System and method for generating extreme ultraviolet light

ABSTRACT

A system includes a chamber, a laser beam apparatus configured to generate a laser beam to be introduced into the chamber, a laser controller for the laser beam apparatus to control at least a beam intensity and an output timing of the laser beam, and a target supply unit configured to supply a target material into the chamber, the target material being irradiated with the laser beam for generating extreme ultraviolet light.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation application of U.S. patentapplication Ser. No. 14/992,506, filed Jan. 11, 2016, which is acontinuation application of U.S. patent application Ser. No. 13/572,484,filed Aug. 10, 2012, which is a continuation-in-part application of U.S.patent application Ser. No. 13/492,067, filed Jun. 8, 2012, which is acontinuation-in-part application of International Patent Application No.PCT/JP2011/052767, filed Feb. 9, 2011, which claims priority fromJapanese Patent Application No. 2010-034889 filed Feb. 19, 2010,Japanese Patent Application No. 2010-265789 filed Nov. 29, 2010,Japanese Patent Application No. 2011-015691 filed Jan. 27, 2011, andJapanese Patent Application No. 2011-133111 filed Jun. 15, 2011. Thepresent application further claims priority from Japanese PatentApplication No. 2012-103580 filed Apr. 27, 2012, and Japanese PatentApplication No. 2012-141079 filed Jun. 22, 2012.

BACKGROUND

1. Technical Field

This disclosure relates to a system and a method for generating extremeultraviolet (EUV) light.

2. Related Art

In recent years, semiconductor production processes have become capableof producing semiconductor devices with increasingly fine feature sizes,as photolithography has been making rapid progress toward finerfabrication. In the next generation of semiconductor productionprocesses, microfabrication with feature sizes at 60 nm to 45 nm, andfurther, microfabrication with feature sizes of 32 nm or less will berequired. In order to meet the demand for microfabrication with featuresizes of 32 nm or less, for example, an exposure apparatus is needed inwhich a system for generating EUV light at a wavelength of approximately13 nm is combined with a reduced projection reflective optical system.

Three kinds of systems for generating EUV light are known in general,which include a Laser Produced Plasma (LPP) type system in which plasmais generated by irradiating a target material with a laser beam, aDischarge Produced Plasma (DPP) type system in which plasma is generatedby electric discharge, and a Synchrotron Radiation (SR) type system inwhich orbital radiation is used to generate plasma.

SUMMARY

A system according to one aspect of this disclosure may include achamber, a laser beam apparatus configured to generate a laser beam tobe introduced into the chamber, a laser controller for the laser beamapparatus to control at least a beam intensity and an output timing ofthe laser beam, and a target supply unit configured to supply a targetmaterial into the chamber. The target material may be irradiated withthe laser beam for generating extreme ultraviolet light.

A system according to another aspect of this disclosure may include achamber, a laser beam apparatus configured to output a laser beam intothe chamber, a laser controller for the laser beam apparatus to controlenergy of the laser beam to achieve a predetermined fluence, and atarget supply unit configured to supply a target material into thechamber. The target material may be irradiated with the laser beam forgenerating extreme ultraviolet light.

A method according to yet another aspect of this disclosure forgenerating extreme ultraviolet light in a system that includes a laserbeam apparatus, a laser controller, a chamber, and a target supply unitmay include supplying a target material into the chamber in a form of adroplet, irradiating the target material with a pre-pulse laser beamfrom the laser beam apparatus, and irradiating the target materialhaving been irradiated with the pre-pulse laser beam with a main pulselaser beam from the laser beam apparatus in a range of 0.5 μs to 3 μsafter the target material is irradiated with the pre-pulse laser beam.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates an exemplary configuration of an EUVlight generation system according to one embodiment of this disclosure.

FIG. 2 is a conceptual diagram showing a droplet being irradiated with apre-pulse laser beam.

FIGS. 3A through 3C show simulation results of diffusion when a moltentin droplet is irradiated with a pre-pulse laser beam.

FIG. 3D is a photograph capturing a molten tin droplet being irradiatedwith a pre-pulse laser beam.

FIG. 4A schematically shows a molten tin droplet being irradiated with apre-pulse laser beam, as viewed in the direction perpendicular to thebeam axis.

FIG. 4B schematically shows a molten tin droplet being irradiated with apre-pulse laser beam, as viewed in the direction of the beam axis.

FIGS. 5A through 5H show the simulation results of diffusion when amolten tin droplet having a diameter of 60 μm is irradiated with apre-pulse laser beam.

FIG. 5I shows the spot size of a main pulse laser beam.

FIG. 6 shows a diffusion diameter of a diffused target generated when amolten tin droplet having a diameter of 60 μm is irradiated with apre-pulse laser beam and a conversion efficiency (CE) corresponding to atiming at which the diffused target is irradiated with a main pulselaser beam.

FIGS. 7A through 7H show the simulation results of diffusion when amolten tin droplet having a diameter of 10 μm is irradiated with apre-pulse laser beam.

FIG. 7I shows the spot size of a main pulse laser beam.

FIG. 8 shows a diffusion diameter of a diffused target generated when amolten tin droplet having a diameter of 10 μm is irradiated with apre-pulse laser beam and a conversion efficiency (CE) corresponding to atiming at which the diffused target is irradiated with a main pulselaser beam.

FIG. 9 schematically illustrates an exemplary configuration of an EUVlight generation system according to a first embodiment.

FIG. 10 schematically illustrates an exemplary configuration of an EUVlight generation system according to a second embodiment.

FIG. 11 schematically illustrates an exemplary configuration of an EUVlight generation system according to a third embodiment.

FIGS. 12A through 12F show a droplet being irradiated with a firstpre-pulse laser beam and a diffused target being irradiated with asecond pre-pulse laser beam.

FIG. 13 schematically illustrates an exemplary configuration of an EUVlight generation system according to a modification of the thirdembodiment.

FIG. 14 schematically illustrates an exemplary configuration of an EUVlight generation system according to a fourth embodiment.

FIG. 15 schematically illustrates an exemplary configuration of aTi:sapphire laser configured to output a pre-pulse laser beam in an EUVlight generation system according to a fifth embodiment.

FIG. 16 schematically illustrates an exemplary configuration of a fiberlaser configured to output a pre-pulse laser beam in an EUV lightgeneration system according to a sixth embodiment.

FIG. 17A is a table showing irradiation conditions of a pre-pulse laserbeam in the EUV light generation system of any one of the embodiments.

FIG. 17B is a table showing irradiation conditions of a main pulse laserbeam in the EUV light generation system of any one of the embodiments.

FIG. 18 schematically illustrates an exemplary configuration of an EUVlight generation system according to a seventh embodiment.

FIG. 19A is a conceptual diagram showing a droplet being irradiated witha linearly-polarized pre-pulse laser beam.

FIG. 19B shows the simulation result of diffusion of the droplet.

FIG. 20A is a conceptual diagram showing a droplet being irradiated witha linearly-polarized pre-pulse laser beam.

FIG. 20B shows the simulation result of diffusion of the droplet.

FIG. 21 is a graph showing absorptivity of a P-polarization componentand an S-polarization component of a laser beam by a molten tin droplet.

FIGS. 22A through 22F show a droplet being irradiated with acircularly-polarized pre-pulse laser beam and a diffused target beingirradiated with a main pulse laser beam according to a seventhembodiment.

FIGS. 23A through 23F show a droplet being irradiated with anunpolarized pre-pulse laser beam and a diffused target being irradiatedwith a main pulse laser beam according to the seventh embodiment.

FIGS. 24A through 24F show a droplet being irradiated with aradially-polarized pre-pulse laser beam and a diffused target beingirradiated with a main pulse laser beam according to the seventhembodiment.

FIGS. 25A through 25F show a droplet being irradiated with anazimuthally-polarized pre-pulse laser beam and a diffused target beingirradiated with a main pulse laser beam according to the seventhembodiment.

FIGS. 26A and 26B are diagrams for discussing a method for measuring thedegree of linear-polarization.

FIG. 27 shows a first example of a polarization converter in the seventhembodiment.

FIGS. 28A through 28C show a second example of a polarization converterin the seventh embodiment.

FIGS. 29A and 29B show a third example of a polarization converter inthe seventh embodiment.

FIG. 30 shows a fourth example of a polarization converter in theseventh embodiment.

FIG. 31 schematically illustrates an exemplary configuration of an EUVlight generation system according to an eighth embodiment.

FIGS. 32A through 32C schematically illustrates an exemplaryconfiguration of a laser apparatus configured to output a pre-pulselaser beam in an EUV light generation system according to a ninthembodiment.

FIGS. 33A through 33C schematically illustrates an exemplaryconfiguration of a laser apparatus configured to output a pre-pulselaser beam in an EUV light generation system according to a modificationof the ninth embodiment.

FIGS. 34A and 34B show an example of a polarization converter in theninth embodiment.

FIG. 35 is a graph on which the obtained conversion efficiency (CE) inaccordance with a fluence of a pre-pulse laser beam is plotted.

FIG. 36 is a graph showing the result of an experiment for generating adiffused target in an EUV light generation system.

FIG. 37 is a graph on which the obtained conversion efficiency (CE) forthe corresponding delay time since a droplet is irradiated with apre-pulse laser beam until a diffused target is irradiated by a mainpulse laser beam is plotted for differing diameters of the droplet.

FIG. 38 is a partial sectional view schematically illustrating anexemplary configuration of an EUV light generation system according to atenth embodiment.

FIG. 39 is a graph showing an example of a relationship between anirradiation condition of a pre-pulse laser beam and a CE in an EUV lightgeneration system.

FIG. 40A is a graph showing an example of a relationship between afluence of a pre-pulse laser beam and a CE in an EUV light generationsystem.

FIG. 40B is a graph showing an example of a relationship between a beamintensity of a pre-pulse laser beam and a CE in an EUV light generationsystem.

FIG. 41A shows photographs of a diffused target generated when a dropletis irradiated with a pre-pulse laser beam in an EUV light generationsystem.

FIG. 41B shows photographs of a diffused target generated when a dropletis irradiated with a pre-pulse laser beam in an EUV light generationsystem.

FIG. 42 schematically illustrates an arrangement of equipment used tocapture the photographs shown in FIGS. 41A and 41B.

FIG. 43A is a sectional view schematically illustrating the diffusedtarget shown in FIG. 41A.

FIG. 43B is a sectional view schematically illustrating the diffusedtarget shown in FIG. 41B.

FIG. 44A is a sectional view schematically illustrating a processthrough which a diffused target is generated when a droplet isirradiated with a pre-pulse laser beam having a pulse duration in thepicosecond range.

FIG. 44B is another sectional view schematically illustrating theprocess through which the diffused target is generated when a droplet isirradiated with a pre-pulse laser beam having a pulse duration in thepicosecond range.

FIG. 44C is yet another sectional view schematically illustrating theprocess through which the diffused target is generated when a droplet isirradiated with a pre-pulse laser beam having a pulse duration in thepicosecond range.

FIG. 45A is a sectional view schematically illustrating a processthrough which a diffused target is generated when a droplet isirradiated with a pre-pulse laser beam having a pulse duration in thenanosecond range.

FIG. 45B is another sectional view schematically illustrating theprocess through which the diffused target is generated when a droplet isirradiated with a pre-pulse laser beam having a pulse duration in thenanosecond range.

FIG. 45C is yet another sectional view schematically illustrating theprocess through which the diffused target is generated when a droplet isirradiated with a pre-pulse laser beam having a pulse duration in thenanosecond range.

FIG. 46 schematically illustrates an exemplary configuration of apre-pulse laser apparatus shown in FIG. 38.

FIG. 47 schematically illustrates an exemplary configuration of amode-locked laser device shown in FIG. 46.

FIG. 48 schematically illustrates an exemplary configuration of aregenerative amplifier shown in FIG. 46.

FIG. 49 schematically illustrates a beam path in the regenerativeamplifier shown in FIG. 48, when a voltage is applied to a Pockels cell.

FIG. 50A is a timing chart of a clock signal in the pre-pulse laserapparatus shown in FIG. 46.

FIG. 50B is a timing chart of a detection signal in the pre-pulse laserapparatus shown in FIG. 46.

FIG. 50C is a timing chart of a first timing signal in the pre-pulselaser apparatus shown in FIG. 46.

FIG. 50D is a timing chart of an AND signal in the pre-pulse laserapparatus shown in FIG. 46.

FIG. 50E is a timing chart of a voltage waveform in the pre-pulse laserapparatus shown in FIG. 46.

FIG. 51 schematically illustrates an exemplary configuration of a mainpulse laser apparatus shown in FIG. 38.

FIG. 52 is a partial sectional view schematically illustrating anexemplary configuration of an EUV light generation system according toan eleventh embodiment.

FIG. 53 schematically illustrates an exemplary configuration of a delaytime control device shown in FIG. 52.

FIG. 54 is a flowchart showing an exemplary operation of a controllershown in FIG. 53.

DETAILED DESCRIPTION

Hereinafter, selected embodiments of this disclosure will be describedin detail with reference to the accompanying drawings. The embodimentsto be described below are merely illustrative in nature and do not limitthe scope of this disclosure. Further, the configuration(s) andoperation(s) described in each embodiment are not all essential inimplementing this disclosure. Note that like elements are referenced bylike reference numerals and characters, and duplicate descriptionsthereof will be omitted herein.

Contents

1. General Configuration

2. Diffusion of Droplet

2.1 Disc-Shaped or Dish-Shaped Diffusion

2.2 Torus-Shaped Diffusion

2.3 Diffusion of Large Droplet

2.4 Diffusion of Small Droplet

3. First Embodiment

4. Second Embodiment

5. Third Embodiment

6. Fourth Embodiment

7. Fifth Embodiment

8. Sixth Embodiment

9. Irradiation Conditions of Laser Beams

10. Seventh Embodiment

10.1 Overview of Polarization Control

10.2 Examples of Polarization Control

10.3 Examples of Polarization Converter

11. Eighth Embodiment

12. Ninth Embodiment

13. Control of Fluence

14. Control of Delay Time

15. Tenth Embodiment

15.1 Configuration

15.2 Operation

15.3 Parameters of Pre-pulse Laser Beam

15.3.1 Relationship between Pulse Duration and CE

15.3.2 Relationship between Pulse Duration and Fluence, and Relationshipbetween Pulse Duration and Beam Intensity

15.3.3 Relationship between Pulse Duration and State of Diffused Target

15.3.4 Generation Process of Diffused Target

15.3.5 Range of Pulse Duration

15.3.6 Range of Fluence

15.4 Pre-pulse Laser Apparatus

15.4.1 General Configuration

15.4.2 Mode-Locked Laser Device

15.4.3 Regenerative Amplifier

15.4.3.1 When Voltage Is Not Applied to Pockels Cell

15.4.3.2 When Voltage Is Applied to Pockels Cell

15.4.4 Timing Control

15.4.5 Examples of Laser Medium

15.5 Main Pulse Laser Apparatus

16. Eleventh Embodiment

1. General Configuration

FIG. 1 schematically illustrates an exemplary configuration of an EUVlight generation system according to an embodiment of this disclosure.The EUV light generation system of this embodiment may be of an LPPtype. As shown in FIG. 1, the EUV light generation system may include achamber 1, a target supply unit 2, a driver laser 3, an EUV collectormirror 5, and an EUV light generation controller 7.

The chamber 1 may be a vacuum chamber, and the EUV light is generatedinside the chamber 1. The chamber 1 may be provided with an exposureapparatus connection port 11 and a window 12. The EUV light generatedinside the chamber 1 may be outputted to an external processingapparatus, such as an exposure apparatus (reduced projection reflectiveoptical system), through the exposure apparatus connection port 11. Alaser beam outputted from the driver laser 3 may enter the chamber 1through the window 12.

The target supply unit 2 may be configured to supply a target material,such as tin (Sn) and lithium (Li), used to generate the EUV light, intothe chamber 1 at a timing specified by a droplet controller 8. Thetarget material inside the target supply unit 2 may be outputted througha target nozzle 13 in the form of droplets DL. The droplet DL may, forexample, be 10 μm to 100 μm inclusive in diameter. Of a plurality ofdroplets DL supplied into the chamber 1, ones that are not irradiatedwith a laser beam may be collected into a target collection unit 14.

The driver laser 3 is configured to output a laser beam used to excitethe target material. The driver laser 3 may be a master oscillator poweramplifier type laser apparatus. The laser beam from the driver laser 3may be a pulse laser beam with a pulse duration of a few to a few tensof nanoseconds and a repetition rate of 10 kHz to 100 kHz. In thisembodiment, the driver laser 3 may be configured to output a pre-pulselaser beam and a main pulse laser beam. As the driver laser 3, acombination of a Yttrium Aluminum Garnet (YAG) laser apparatus foroutputting a pre-pulse laser beam and a CO₂ laser apparatus foroutputting a main pulse laser beam may be used. However, this embodimentis not limited thereto, and any suitable laser apparatus may be used.

Each of the pre-pulse laser beam and the main pulse laser beam from thedriver laser 3 may be reflected by a laser beam focusing optical systemthat includes a high-reflection mirror 15 a and an off-axis paraboloidalmirror 15 b, and enter the chamber 1 through the window 12. Inside thechamber 1, each of the pre-pulse laser beam and the main pulse laserbeam may be focused in a plasma generation region PS.

When the droplet DL is irradiated with the pre-pulse laser beam, thedroplet DL may be diffused into fine particles. In this specification, atarget material in a state where fine particles of a droplet DL arediffused may be referred to as a diffused target. The diffused targetmay be irradiated with the main pulse laser beam. Upon being irradiatedwith the main pulse laser beam, the target material constituting thediffused target may be excited by the energy of the main pulse laserbeam. With this, the target material may be turned into plasma, and raysof light at various wavelengths including the EUV light may be emittedfrom the plasma.

The EUV collector mirror 5 may be configured to selectively reflectlight at a predetermined wavelength (e.g., EUV light at a centralwavelength of approximately 13.5 nm) among rays of light at variouswavelengths emitted from the plasma. The EUV collector mirror 5 may havea spheroidal concave surface on which a multilayer reflective filmformed of a molybdenum (Mo) layer and a silicon (Si) layer laminatedalternately is formed. The EUV collector mirror 5 may be positioned suchthat a first focus of the spheroidal surface lies in the plasmageneration region PS and a second focus thereof lies in an intermediatefocus region IF. With this, the EUV light reflected by the EUV collectormirror 5 may be focused at the second focus, and outputted to anexternal exposure apparatus.

The EUV light generation controller 7 may be configured to output anoscillation trigger signal and a laser beam intensity setting signal tothe driver laser 3. With this, the EUV light generation controller 7 maycontrol the beam intensity and the generation timing of the pre-pulselaser beam such that a droplet supplied into the chamber 1 istransformed into a desired diffused target. Further, the EUV lightgeneration controller 7 may control the beam intensity and thegeneration timing of the main pulse laser beam such that plasma in adesired condition may be generated from the diffused target upon beingirradiated with the main pulse laser beam.

The oscillation trigger signal may be outputted based on an oscillationtrigger detection signal from an exposure apparatus controller 9, andthe generation timing of the laser beams by the driver laser 3 may becontrolled accordingly. The laser beam intensity setting signal may beoutputted based on the oscillation trigger detection signal from theexposure apparatus controller 9 and an EUV pulse energy detection signalfrom either an EUV light detector 16 or the exposure apparatuscontroller 9. The laser beam intensity setting signal may be outputtedto the driver laser 3 in order to control the beam intensity of thelaser beams. The EUV light generation controller 7 may include a triggercounter 7 a and a timer 7 b, and may count the number of oscillationtrigger detection signals per unit time. The laser beam intensitysetting signal may be outputted based on the EUV pulse energy detectionsignal and the number of counted oscillation trigger detection signals.

2. Diffusion of Droplet

Diffusion of a droplet upon being irradiated with a pre-pulse laser beamwill now be discussed. FIG. 2 is a conceptual diagram showing a dropletbeing irradiated with a pre-pulse laser beam. In FIG. 2, the droplet isviewed in a direction perpendicular to the beam axis (Z-direction) ofthe pre-pulse laser beam.

As shown in FIG. 2, when the pre-pulse laser beam is focused on thedroplet DL, laser ablation may occur at a surface of the droplet DL thathas been irradiated with the pre-pulse laser beam. As a result, a shockwave or sonic wave may occur from the surface of the droplet DLirradiated with the pre-pulse laser beam toward the interior of thedroplet DL due to the energy by the laser ablation. This shock wave orsonic wave may propagate throughout the droplet DL. The droplet DL maynot be broken up when the beam intensity of the pre-pulse laser beam isweak. However, when the beam intensity of the pre-pulse laser beam isequal to or greater than a first predetermined value (e.g., 1×10⁹W/cm²), the droplet DL may be broken up.

2.1 Disc-Shaped or Dish-Shaped Diffusion

FIGS. 3A through 3C show the simulation results of diffusion of a moltentin droplet being irradiated with a pre-pulse laser beam. FIG. 3D is aphotograph capturing a molten tin droplet being irradiated with apre-pulse laser beam under the condition that is identical to that inthe simulation shown in FIG. 3C. In each of FIGS. 3A through 3D, thedroplet is viewed in a direction perpendicular to the beam axis of thepre-pulse laser beam (Z-direction). Further, in FIGS. 3A through 3C, thespot size of the main pulse laser beam and the beam intensity of thepre-pulse laser beam striking the droplet DL are indicated. In FIG. 3B,a diffusion diameter De of the diffused target and an irradiation spotsize Dm of the main pulse laser beam are indicated.

As shown in FIG. 3A, when the beam intensity of the pre-pulse laser beamis 6.4×10⁸ W/cm², the droplet is hardly diffused. On the other hand, asshown in FIG. 3B, when the beam intensity of the pre-pulse laser beam is1.6×10⁹ W/cm² (2.5 times greater than the beam intensity in thesimulation shown in FIG. 3A), the droplet is broken up. The broken-updroplet is turned into numerous minute particles and forms a diffusedtarget. These minute particles may be diffused in a disc-shape as viewedin the Z-direction. Further, as shown in FIG. 3C, when the beamintensity of the pre-pulse laser beam is 5.5×10⁹ W/cm² (8.6 timesgreater than the beam intensity in the simulation shown in FIG. 3A), thedroplet is broken up, and the minute particles of the broken-up dropletmay be diffused in a dish-shape. As can been seen from the comparisonbetween FIG. 3C and FIG. 3D, the state of the actual diffusion of theminute particles were similar to the simulation result.

In the case shown in FIG. 3A, it is speculated that even when thedroplet is irradiated with the main pulse laser beam, a large portion ofthe energy of the main pulse laser beam is not absorbed by the droplet,whereby a high CE may not be obtained. That is, with respect to the sizeof the target material after being irradiated with the pre-pulse laserbeam, the irradiation spot size of the main pulse laser beam is toolarge. Accordingly, a large portion of the main pulse laser beam may notstrike the droplet and may not be used to generate plasma. On the otherhand, in the cases shown in FIGS. 3B and 3C, the droplet is diffused inthe irradiation spot of the main pulse laser beam, whereby a largeportion of the main pulse laser beam may be used to generate plasma.Further, a diffused target may have a greater total surface area than asingle droplet. As shown below, when a single droplet is broken into n³smaller pieces, the radius of a smaller piece may become (1/n) of theradius of the original droplet. Here, the total surface area of thesmaller pieces may be n times greater than the surface area of theoriginal droplet.

When the radius of an undiffused droplet is r, a volume V₁ of theundiffused droplet may be expressed in Expression (1) below.V ₁=4πr ³/3  (1)

A total volume V₂ of n³ smaller pieces each having a radius (r/n) may beexpressed in Expression (2) below.V ₂ =n ³×4π(r/n)³/3  (2)

The total volume V₂ of n³ smaller pieces each having the radius (r/n)may be equal to the volume V₁ of the undiffused droplet having theradius r (V₂=V₁).

A surface area S₁ of the undiffused droplet having the radius r may beexpressed in Expression (3) below.S ₁=4πr ²  (3)

A total surface area S₂ of n³ smaller pieces each having the radius(r/n) may be expressed in Expression (4) below.S ₂ =n ³×4π(r/n)² =n×4πr ²  (4)

Accordingly, the total surface area S₂ of n³ smaller pieces each havingthe radius (r/n) is n times greater than the surface area S₁ of theundiffused droplet having the radius r.

In this way, in the cases shown in FIGS. 3B and 3C, the droplet may bediffused, and the total surface area may be increased. As a result, theenergy of the main pulse laser beam may be absorbed efficiently by thediffused small particles. With this, a larger portion of the diffusedsmall particles may be turned into plasma, and EUV light with higherenergy may be obtained. Accordingly, a high CE may be obtained.

In either of the cases shown in FIGS. 3B and 3C, the diffused target hassuch a shape that the length in the direction of the beam axis of thepre-pulse laser beam is shorter than the length in the directionperpendicular to the beam axis of the pre-pulse laser beam. The diffusedtarget having such a shape may be irradiated with the main pulse laserbeam traveling substantially along the same path as the pre-pulse laserbeam. Since the diffused target may be irradiated with the main pulselaser beam more uniformly, the main pulse laser beam may be absorbedefficiently by the target material.

The diffusion diameter De of the diffused target may be equal to orsmaller than the irradiation spot size Dm of the main pulse laser beam.Because of this size, the entire diffused target may be irradiated withthe main pulse laser beam, and thus a larger portion of the diffusedtarget may be turned into plasma. As a result, generation of debris ofthe target material may be suppressed.

Further, the diffusion diameter De of the diffused target may be equalto or closer to the irradiation spot size Dm of the main pulse laserbeam. With this, a larger portion of the energy of the main pulse laserbeam may be absorbed by the diffused target, whereby a higher CE may beobtained. Although FIG. 3B shows that the position of the beam waist ofthe main pulse laser beam substantially coincides with the position ofthe diffused target, this disclosure is not limited thereto. That is,the position of the beam waist of the main pulse laser beam and theposition of the diffused target do not necessarily have to coincide witheach other. In this disclosure, the irradiation spot size Dm may beinterpreted as a diameter of a cross-section of the main pulse laserbeam at or around the position at which the diffused target isirradiated with the main pulse laser beam.

Although a case where the main pulse laser beam has a circularcross-section and the cross-section of the diffused target is circularhas been described, this disclosure is not limited thereto. For example,a cross-section area of the main pulse laser beam may be larger than amaximum cross-section area of the diffused target.

2.2 Torus-Shaped Diffusion

FIGS. 4A and 4B schematically show a molten tin droplet having beenirradiated with the pre-pulse laser beam. In FIG. 4A, the diffusedtarget is viewed in a direction perpendicular to the beam axes of thepre-pulse laser beam and the main pulse laser beam (Z-direction). InFIG. 4B, the diffused target is viewed in a direction of the beam axesof the pre-pulse laser beam and the main pulse laser beam. In FIG. 4B,an outer diameter Dout of a torus-shaped diffused target and theirradiation spot size Dm of the main pulse laser beam are indicated.

As described with reference to FIG. 2, when the pre-pulse laser beam isfocused on the droplet DL, laser ablation may occur at the surface ofthe droplet DL. Here, when the beam intensity of the pre-pulse laserbeam is equal to or greater than a second predetermined value (e.g.,6.4×10⁹ W/cm²), the droplet DL may be broken up, and a torus-shapeddiffused target as shown in FIGS. 4A and 4B may be formed. Thetorus-shaped diffused target may be diffused symmetrically about thebeam axis of the pre-pulse laser beam and into a torus-shape.

For example, for generating a torus-shaped diffused target, the beamintensity of the pre-pulse laser beam may be in the range of 6.4×10⁹W/cm² to 3.2×10¹⁰ W/cm² inclusive, and the diameter of the droplet maybe in the range of 12 μm and 40 μm inclusive.

Irradiation of the torus-shaped diffused target with the main pulselaser beam will now be described. A torus-shaped diffused target may beformed in 0.5 μs to 2.0 μs after a droplet is irradiated with apre-pulse laser beam. Accordingly, the diffused target may be irradiatedwith the main pulse laser beam in the above time span after the dropletis irradiated with the pre-pulse laser beam.

Further, as shown in FIGS. 4A and 4B, the torus-shaped diffused targethas such a shape that the length in the direction of the beam axis ofthe pre-pulse laser beam is shorter than the length in the directionperpendicular to the beam axis of the pre-pulse laser beam. The diffusedtarget having such a shape may be irradiated with the main pulse laserbeam traveling substantially along the same path as the pre-pulse laserbeam. With this, the diffused target may be irradiated with the mainpulse laser beam more efficiently, and thus the main pulse laser beammay be absorbed efficiently by the target material. Accordingly, the CEin the LPP type EUV light generation system may be improved. The CE ofapproximately 3% was obtained through an experiment under the aboveconditions.

For example, it is speculated that when a torus-shaped diffused targetis irradiated with a main pulse laser beam of a Gaussian beam intensitydistribution, plasma is emitted cylindrically from the torus-shapeddiffused target. Then, the plasma diffused toward the inner portion ofthe cylinder may be trapped therein. Accordingly, high-temperature,high-density plasma may be generated, and the CE may be improved. Here,“torus-shape” means an annular shape, but the diffused target does notnecessarily have to be perfectly annular in shape, and may besubstantially annular in shape.

Further, the irradiation spot size Dm of the main pulse laser beam maybe in the following relationship with the outer diameter Dout of thetorus-shaped diffused target.Dm≧DoutWith this relationship, the entire torus-shaped diffused target may beirradiated with the main pulse laser beam, and a larger portion of thediffused target may be turned into plasma. As a result, generation ofdebris of the target material may be reduced.2.3 Diffusion of Large Droplet

FIGS. 5A through 5H show the simulation result of diffusion when amolten tin droplet having a diameter of 60 μm is irradiated with apre-pulse laser beam. In each of FIGS. 5A through 5D, the droplet or thediffused target is viewed in a direction (X-direction) perpendicular tothe beam axis of the pre-pulse laser beam (Z-direction). FIGS. 5Athrough 5D respectively show the states of the target material attimings where a time T is 0 μs, 0.4 μs, 0.8 μs, and 1.4 μs after thedroplet DL is irradiated with the pre-pulse laser beam. In each of FIGS.5E through 5H, the droplet or the diffused target is viewed in thedirection (Z-direction) of the beam axis of the pre-pulse laser beam.FIGS. 5E through 5H respectively show the states of the target materialat timings where a time T is 0 μs, 0.4 μs, 0.8 μs, and 1.4 μs after thedroplet DL is irradiated with the pre-pulse laser beam. FIG. 5I showsthe irradiation spot size of the main pulse laser beam at a positionwhere the diffused target is irradiated with the main pulse laser beam.Here, the beam intensity of the pre-pulse laser beam is 1.5×10⁹ W/cm².

With reference to the simulation results shown in FIGS. 5A through 5Halong with the irradiation spot size of the main pulse laser beam shownin FIG. 5I, the following can be found. A large portion of the diffusedtarget may be irradiated with the main pulse laser beam in approximately0.4 μs after a droplet is irradiated with the pre-pulse laser beam.Accordingly, generation of debris may be reduced if the diffused targetis irradiated with the main pulse laser beam at the above timing.

A droplet having a diameter of 60 μm may be broken into small particlesand diffused upon being irradiated with a pre-pulse laser beam. In eachof FIGS. 5A through 5D, the maximum value and the minimum value of adiameter of a small particle in the diffused target are indicated. Withthe beam intensity of the pre-pulse laser beam in this simulation, themaximum value of a diameter of a small particle in the diffused targetis 48.0 μm. That is, the droplet has not been broken up sufficiently bythe pre-pulse laser beam, and a large portion of the diffused target maynot be turned into plasma even when the diffused target is irradiatedwith the main pulse laser beam. This may suggest that a large amount ofdebris may be generated. The minimum value of a diameter of a smallparticle in the diffused target is 3.7 μm in 0.4 μs, 3.5 μm in 0.8 μs,and 3.1 μm in 1.4 μs, respectively, after a droplet is irradiated with apre-pulse laser beam. This suggests that the more the time T elapsesafter a droplet is irradiated with a pre-pulse laser beam, the smallerthe diameter of a small particle becomes, and the number of smallparticles may increase. This in turn suggests that in the case where amolten tin droplet having a diameter of 60 μm is irradiated with apre-pulse laser beam, if a diffused target is irradiated with a mainpulse laser beam within a range where the time T after the droplet isirradiated with the pre-pulse laser beam is between 0.4 μs and 1.4 μs,the CE may be improved further with a longer time T.

FIG. 6 shows a change over time in the diffusion diameter De of thediffused target when a molten tin droplet having a diameter of 60 μm isirradiated with the pre-pulse laser beam and a conversion efficiencywhen the diffused target is irradiated with a main pulse laser beam at agiven point in time. As shown in FIGS. 5F and 6, the diffusion diameterDe of the diffused target may substantially coincide with theirradiation spot size of the main pulse laser beam in approximately 0.4μs after the droplet is irradiated with the pre-pulse laser beam.Accordingly, generation of debris may be reduced if the diffused targetis irradiated with the main pulse laser beam in 0.4 μs after the dropletis irradiated with the pre-pulse laser beam (see a white arrow A in FIG.6). On the other hand, with reference to FIG. 6, a high CE may beobtained if the diffused target is irradiated with the main pulse laserbeam in approximately 3 μs after the droplet is irradiated with thepre-pulse laser beam (see a white arrow B in FIG. 6). This simulationresults suggest that a delay time for the main pulse laser beam from theirradiation with the pre-pulse laser beam to reduce generation of debrismay differ from a delay time to obtain a high CE. That is, when a moltentin droplet having a diameter of 60 μm is irradiated sequentially with apre-pulse laser beam and then a main pulse laser beam, it may bedifficult to reduce debris and obtain a high CE at the same time.

2.4 Diffusion of Small Droplet

FIGS. 7A through 7H show the simulation results of diffusion when amolten tin droplet having a diameter of 10 μm is irradiated with thepre-pulse laser beam. In each of FIGS. 7A through 7D, the droplet or thediffused target is viewed in a direction (X-direction) perpendicular tothe beam axis of the pre-pulse laser beam (Z-direction). FIGS. 7Athrough 7D respectively show the states of the target material attimings where a time T is 0 μs, 0.1 μs, 0.25 μs, and 0.5 μs after thedroplet is irradiated with the pre-pulse laser beam. In each of FIGS. 7Ethrough 7H, the droplet or the diffused target is viewed in thedirection of the beam axis of the pre-pulse laser beam (Z-direction).FIGS. 7E through 7H respectively show the states of the target materialat timings where a time T is 0 μs, 0.1 μs, 0.25 μs, and 0.5 μs after thedroplet is irradiated with the pre-pulse laser beam. FIG. 7I shows theirradiation spot size of the main pulse laser beam at a position wherethe diffused target is irradiated with the main pulse laser beam. Here,the beam intensity of the pre-pulse laser beam is 1.5×10⁹ W/cm².

With reference to the simulation results shown in FIGS. 7A through 7Halong with the irradiation spot size of the main pulse laser beam shownin FIG. 7I, it can be said that a large portion of the diffused targetmay be irradiated with the main pulse laser beam in 0.1 μs after thedroplet is irradiated with the pre-pulse laser beam. Accordingly,generation of debris may be reduced if the diffused target is irradiatedwith the main pulse laser beam at the above timing.

As shown in FIGS. 7A through 7D, the maximum value of a diameter of asmall particle in a diffused target is 2.2 μm in 0.1 μs, 1.1 μm in 0.25μs, and 1.1 μs in 0.5 μs after the droplet is irradiated with thepre-pulse laser beam. This suggests that the maximum value of a diameterof a small particle in a diffused target becomes constant in 0.25 μsafter the droplet is irradiated with the pre-pulse laser beam. Theminimum value of a diameter of a small particle in the diffused targetis 0.2 μm in 0.1 μs, 0.2 μm in 0.25 μs, and 0.2 μm in 0.5 μs after thedroplet is irradiated with the pre-pulse laser beam. This suggests thata small particle in a diffused target is sufficiently small in 0.1 μsafter the droplet is irradiated with the pre-pulse laser beam. This inturn suggests that a higher CE may be obtained if the diffused target isirradiated with the main pulse laser beam in 0.1 μs after the droplet isirradiated with the pre-pulse laser beam.

FIG. 8 shows a change over time in the diffusion diameter De of thediffused target when a molten tin droplet having a diameter of 10 μm isirradiated with the pre-pulse laser beam and a conversion efficiencywhen the diffused target is irradiated with the main pulse laser beam ata given point in time.

As shown in FIGS. 7F and 8, the diffusion diameter De of the diffusedtarget may substantially coincide with the irradiation spot size of themain pulse laser beam in 0.1 μs after the droplet is irradiated with thepre-pulse laser beam. Accordingly, generation of debris may be reducedif the diffused target is irradiated with the main pulse laser beam in0.1 μs after the droplet is irradiated with the pre-pulse laser beam(see a white arrow A in FIG. 8). On the other hand, with reference toFIG. 8, a high CE may be obtained if the diffused target is irradiatedwith the main pulse laser beam in approximately 0.15 μs after thedroplet is irradiated with the pre-pulse laser beam (see a white arrow Bin FIG. 8). The simulation results suggest that a gap between a delaytime for the main pulse laser beam to reduce debris and a delay time forthe main pulse laser beam to obtain a high CE is relatively small. Thatis, when a molten tin droplet having a diameter of 10 μm is irradiatedsequentially with the pre-pulse laser beam and then the main pulse laserbeam, it may be possible to reduce debris and obtain a high CE at thesame time. A molten tin droplet having a diameter of 10 μm may bereferred to as a mass-limited target since it is a target with a minimummass required for generating desired EUV light.

3. First Embodiment

FIG. 9 schematically illustrates an exemplary configuration of an EUVlight generation system according to a first embodiment. In the EUVlight generation system according to the first embodiment, a beam pathof a pre-pulse laser beam from a YAG pulse laser apparatus 3 a and abeam path of a main pulse laser beam from a CO₂ pulse laser apparatus 3b may be made to substantially coincide with each other by a beamcombiner 15 c. That is, in the first embodiment, the pre-pulse laserbeam and the main pulse laser beam are guided into the chamber 1 alongsubstantially the same path.

First, an EUV light emission signal may be inputted to the EUV lightgeneration controller 7 from the exposure apparatus controller 9. TheEUV light generation controller 7 may be configured to output a YAGlaser beam intensity setting signal to the YAG pulse laser apparatus 3a. Further, the EUV light generation controller 7 may be configured tooutput a CO₂ laser beam intensity setting signal to the CO₂ pulse laserapparatus 3 b.

In addition, the EUV light generation controller 7 may be configured tooutput an EUV light emission trigger signal to a trigger controller 17.The trigger controller 17 may be configured to output a droplet outputsignal to a droplet controller 8. The droplet controller 8 may input thedroplet output signal to the target supply unit 2, and upon receivingthe droplet output signal, the target supply unit 2 may output a dropletDL through the target nozzle 13. The trigger controller 17 may beconfigured to output a YAG laser oscillation trigger signal to the YAGpulse laser apparatus 3 a. The YAG laser oscillation trigger signal maybe outputted such that the droplet DL is irradiated with the pre-pulselaser beam at a timing at which the droplet DL reaches the plasmageneration region PS. Further, the trigger controller 17 may beconfigured to output a CO₂ laser oscillation trigger signal to a masteroscillator 3 d in the CO₂ pulse laser apparatus 3 b. The CO₂ laseroscillation trigger signal may be outputted such that the diffusedtarget is irradiated with the main pulse laser beam after a delay time Tfrom the timing at which the droplet DL is irradiated with the pre-pulselaser beam. Here, the delay time T is a time required for a desireddiffused target to be formed.

The YAG pulse laser apparatus 3 a may be configured to output thepre-pulse laser beam at a first wavelength based on the YAG laser beamintensity setting signal from the EUV light generation controller 7 andthe YAG laser oscillation trigger signal from the trigger controller 17.The pre-pulse laser beam from the YAG pulse laser apparatus 3 a may beexpanded in diameter by a beam expander 4 and then be incident on thebeam combiner 15 c.

The CO₂ pulse laser apparatus 3 b may include the master oscillator 3 d,a preamplifier 3 h, a main amplifier 3 j, and relay optical systems 3 g,3 i, and 3 k respectively disposed downstream from the master oscillator3 d, the preamplifier 3 h, and the main amplifier 3 j. The masteroscillator 3 d may be configured to output a seed beam at a secondwavelength based on the CO₂ pulse laser oscillation trigger signal. Theseed beam from the master oscillator 3 d may be amplified to a desiredbeam intensity by the preamplifier 3 h and the main amplifier 3 j basedon the CO₂ laser beam intensity setting signal. The amplified laser beammay be outputted from the CO₂ pulse laser apparatus 3 b as the mainpulse laser beam and be incident on the beam combiner 15 c.

The beam combiner 15 c may be configured to transmit the pre-pulse laserbeam at the first wavelength (e.g., 1.06 μm) and reflect the main pulselaser beam at the second wavelength (e.g., 10.6 μm). More specifically,the beam combiner 15 c may include a diamond substrate on which amultilayer film having the aforementioned reflection/transmissionproperties for the pre-pulse laser and the main pulse laser is formed.Accordingly, the beam combiner 15 c may serve to make the beam path ofthe pre-pulse laser beam and the beam path of the main pulse laser beamcoincide with each other and supply the pre-pulse laser beam and themain pulse laser beam into the chamber 1 along the same path.Alternatively, a beam combiner configured to reflect the pre-pulse laserbeam at the first wavelength and transmit the main pulse laser beam atthe second wavelength may be used to make the respective beam pathscoincide with each other.

The droplet controller 8, the YAG pulse laser apparatus 3 a, and the CO₂pulse laser apparatus 3 b may operate in synchronization with oneanother based on the various signals from the trigger controller 17.With this, the YAG pulse laser apparatus 3 a may output the pre-pulselaser beam in synchronization with the timing at which the dropletsupplied into the chamber 1 from the target supply unit 2 reaches apredetermined region. Then, the CO₂ pulse laser apparatus 3 b may outputthe main pulse laser beam in synchronization with the timing at which adesired diffused target is formed after the droplet is irradiated withthe pre-pulse laser beam.

According to the first embodiment, the pre-pulse laser beam and the mainpulse laser beam may be guided to the plasma generation region PS insubstantially the same direction (substantially the same path). Thus, athrough-hole formed in the EUV collector mirror 5 may be made small andneed not be formed in plurality.

Further, the wavelength (e.g., 1.06 μm) of the pre-pulse laser beam fromthe YAG pulse laser apparatus 3 a is equal to or shorter than one-tenthof the wavelength (e.g., 10.6 μm) of the main pulse laser beam from theCO₂ pulse laser apparatus 3 b. When the wavelength of the pre-pulselaser beam is sufficiently shorter than the wavelength of the main pulselaser beam, the following advantages may be speculated.

(1) The absorptivity of the pre-pulse laser beam by the target material,such as tin, may be higher than that of the main pulse laser beam.

(2) The irradiation spot size of the pre-pulse laser beam focused on thedroplet may be reduced.

As a result, a small droplet DL may be irradiated efficiently with thepre-pulse laser beam having small pulse energy and be diffused.

4. Second Embodiment

FIG. 10 schematically illustrates an exemplary configuration of an EUVlight generation system according to a second embodiment. In the EUVlight generation system according to the second embodiment, thepre-pulse laser beam from the YAG pulse laser apparatus 3 a and the mainpulse laser beam from the CO₂ pulse laser apparatus 3 b are guided intothe chamber 1 along separate beam paths.

The pre-pulse laser beam outputted from the YAG pulse laser apparatus 3a may be reflected by a high-reflection mirror 15 e and an off-axisparaboloidal mirror 15 g. Then, the pre-pulse laser beam may passthrough a through-hole formed in the EUV collector mirror 5, and befocused on a droplet inside the chamber 1 to form a diffused target.

The main pulse laser beam outputted from the CO₂ pulse laser apparatus 3b may be reflected by a high-reflection mirror 15 d and an off-axisparaboloidal mirror 15 f. Then, the main pulse laser beam may passthrough another through-hole formed in the EUV collector mirror 5, andbe focused on the diffused target inside the chamber 1.

According to the second embodiment, the pre-pulse laser beam and themain pulse laser beam may be guided through separate optical systems tothe plasma generation region PS. Accordingly, each of the pre-pulselaser beam and the main pulse laser beam may be focused to have adesired beam spot with ease. Further, an optical element, such as a beamcombiner, for making the beam paths of the pre-pulse laser beam and themain pulse laser beam need not be used. Still, the pre-pulse laser beamand the main pulse laser beam may strike the droplet DL and the diffusedtarget respectively in substantially the same direction.

5. Third Embodiment

FIG. 11 schematically illustrates an exemplary configuration of an EUVlight generation system according to a third embodiment. In the EUVlight generation system according to the third embodiment, a firstpre-pulse laser beam from the YAG pulse laser apparatus 3 a and a secondpre-pulse laser beam and the main pulse laser beam from the CO₂ pulselaser apparatus 3 b may be guided into the chamber 1.

The CO₂ pulse laser apparatus 3 b may include the master oscillator 3 dconfigured to output the seed beam of the main pulse laser beam and amaster oscillator 3 e configured to output a seed beam of the secondpre-pulse laser beam. The seed beam of the second pre-pulse laser beamfrom the master oscillator 3 e may be amplified by the preamplifier 3 hand the main amplifier 3 j to desired beam intensity. The amplified seedbeam may be outputted from the CO₂ pulse laser apparatus 3 b as thesecond pre-pulse laser beam, and then be incident on the beam combiner15 c. The seed beam of the main pulse laser beam from the masteroscillator 3 d may also be amplified by the preamplifier 3 h and themain amplifier 3 j to desired beam intensity. The amplified seed beammay be outputted from the CO₂ pulse laser apparatus 3 b as the mainpulse laser beam, and then be incident on the beam combiner 15 c.

Each of the master oscillators 3 d and 3 e may be a semiconductor laserconfigured to oscillate in a bandwidth that can be amplified by a CO₂gain medium. More specifically, each of the master oscillators 3 d and 3e may include a plurality of quantum cascade lasers (QCL).

FIGS. 12A through 12F show a droplet DL being irradiated with a firstpre-pulse laser beam and a diffused target being irradiated with asecond pre-pulse laser beam in the third embodiment. In each of FIGS.12A through 12C, the droplet or the diffused target is viewed in adirection (X-direction) perpendicular to the beam axes of the first andsecond pre-pulse laser beams (Z-direction). FIGS. 12A through 12Crespectively show the states of the target material at delay times T=0,T=t2, and T=tm (where, 0<t2<tm) after the droplet is irradiated with thefirst pre-pulse laser beam. In each of FIGS. 12D through 12F, thedroplet or the diffused target is viewed in the direction of the beamaxes of the first and second pre-pulse laser beams (Z-direction). FIGS.12D through 12F respectively show the states of the target material atdelay times T=0, T=t2, and T=tm (where, 0<t2<tm) after the droplet isirradiated with the first pre-pulse laser beam.

When a droplet of the target material shown in FIGS. 12A and 12D isirradiated with the first pre-pulse laser beam, the droplet may bediffused as shown in FIGS. 12B and 12E so that a first diffused targetmay be formed. The first diffused target may be irradiated with thesecond pre-pulse laser beam when the first diffused target is diffusedto a desired size that is substantially the same as or smaller than theirradiation spot size of the second pre-pulse laser beam.

When the first diffused target is irradiated with the second pre-pulselaser beam, the first diffused target may be broken into even smallerparticles and be diffused to form a second diffused target. The seconddiffused target may be irradiated with the main pulse laser beam whenthe second diffused target is diffused to a desired size that issubstantially the same as or smaller than the irradiation spot size ofthe main pulse laser beam.

Since the second diffused target, which includes smaller particles thanthose in the first diffused target, is irradiated with the main pulselaser beam, the energy of the main pulse laser beam may be absorbed bythe second diffused target efficiently. Because a large portion of thesecond diffused target may be turned into plasma, a high CE may beobtained. Further, by controlling the irradiation spot size of the mainpulse laser beam to substantially coincide with the diffusion diameterof the second diffused target, a high CE and debris reduction may bothbe achieved.

Note that, in the third embodiment, a mass limited target (e.g., amolten tin droplet having a diameter of 10 μm) may be used.

In the third embodiment, the target material is irradiated with thefirst and second pre-pulse laser beams, and then the diffused target isirradiated with the main pulse laser beam. However, this disclosure isnot limited thereto, and the target material may be irradiated withthree or more pre-pulse laser beams.

Further, in the third embodiment, the first pre-pulse laser beam isoutputted from the YAG pulse laser apparatus 3 a, and the secondpre-pulse laser beam and the main pulse laser beam are outputted fromthe CO₂ pulse laser apparatus 3 b. However, this disclosure is notlimited thereto, and all the laser beams may be outputted, for example,from a CO₂ laser apparatus.

Alternatively, the first and second pre-pulse laser beams may beoutputted from a first laser apparatus, and the main pulse laser beammay be outputted from a second laser apparatus. Here, the first laserapparatus may be a YAG laser apparatus or a fiber laser apparatus, andthe second laser apparatus may be a CO₂ laser apparatus.

FIG. 13 schematically illustrates an exemplary configuration of an EUVlight generation system according to a modification of the thirdembodiment. The EUV light generation system shown in FIG. 13 may includea first YAG pulse laser apparatus 3 m, a second YAG pulse laserapparatus 3 n, and a beam combiner 3 p.

The first and second YAG pulse laser apparatuses 3 m and 3 n may eachreceive the YAG laser beam intensity setting signal from the EUV lightgeneration controller 7 and the YAG laser oscillation trigger signalfrom the trigger controller 17. The first YAG pulse laser apparatus 3 mmay be configured to output the first pre-pulse laser beam, and thefirst pre-pulse laser beam may be incident on the beam combiner 3 p. Thesecond YAG pulse laser apparatus 3 n may be configured to output thesecond pre-pulse laser beam, and the second pre-pulse laser beam mayalso be incident on the beam combiner 3 p. The beam combiner 3 p may bepositioned to make the beam paths of the first and second pre-pulselaser beams coincide with each other and output the first and secondpre-pulse laser beams toward the beam expander 4.

Even with this configuration, as in the third embodiment described withreference to FIG. 11, the first and second pre-pulse laser beams and themain pulse laser beam may be guided into the chamber 1. Here, the firstand second pre-pulse laser beams may respectively be outputted fromfirst and second fiber laser apparatuses.

6. Fourth Embodiment

FIG. 14 schematically illustrates an exemplary configuration of an EUVlight generation system according to a fourth embodiment. FIG. 14 showsa sectional view taken along XIV-XIV plane in any of FIGS. 9 through 11and 13. An EUV light generation system according to the fourthembodiment may be similar in configuration to any one of the firstthrough third embodiments but may differ in that the EUV lightgeneration system of the fourth embodiment may further include magnets 6a and 6 b. A magnetic field may be generated with the magnets 6 a and 6b inside the chamber 1 and ions generated inside the chamber 1 may becollected by the magnetic field.

Each of the magnets 6 a and 6 b may be an electromagnet that includes acoil winding and a cooling mechanism of the coil winding. A power source6 c that is controlled by a power source controller 6 d may be connectedto each of the magnets 6 a and 6 b. The power source controller 6 d mayregulate current to be supplied to the magnets 6 a and 6 b from thepower source 6 c so that a magnetic field in a predetermined directionmay be generated in the chamber 1. A superconductive magnet, forexample, may be used as each of the magnets 6 a and 6 b. Although twomagnets 6 a and 6 b are used in this embodiment, a single magnet may beused. Alternatively, a permanent magnet may be provided in the chamber1.

Plasma generated when a target material is irradiated with a main pulselaser beam may include positive ions and negative ions (or electrons).The positive and negative ions moving inside the chamber 1 may besubjected to Lorentz force in the magnetic field, and thus the ions maymove in spiral along magnetic lines of force. With this, the ionizedtarget material may be trapped in the magnetic field and collected intoion collection units 19 a and 19 b provided in the magnetic field.Accordingly, debris inside the chamber 1 may be reduced, anddeterioration in optical element, such as the EUV collector mirror 5,due to the debris adhering to the optical element may be suppressed. InFIG. 14, the magnetic field is in the direction shown by an arrow, but asimilar function can be achieved even when the magnetic field isoriented in the opposite direction.

A mitigation technique for reducing debris adhering to the opticalelement is not limited to the use of the magnetic field. Alternatively,a substance deposited onto the EUV collector mirror 5 may be etchedusing an etching gas. Debris may be made to react with hydrogen gas (H₂)or a hydrogen radical (H) in the magnetic field, and the debris may beremoved as a vaporized compound.

7. Fifth Embodiment

FIG. 15 schematically illustrates an exemplary configuration of aTi:sapphire laser configured to output the pre-pulse laser beam in anEUV light generation system according to a fifth embodiment. ATi:sapphire laser 50 a of the fifth embodiment may be provided outsidethe chamber 1 as a driver laser for outputting the pre-pulse laser beamin any one of the first through fourth embodiments.

The Ti:sapphire laser 50 a may include a laser resonator formed by asemiconductor saturable absorber mirror 51 a and an output coupler 52 a.A concave mirror 53 a, a first pumping mirror 54 a, a Ti:sapphirecrystal 55 a, a second pumping mirror 56 a, and two prisms 57 a and 58 aare provided in this order from the side of the semiconductor saturableabsorber mirror 51 a in the optical path in the laser resonator.Further, the Ti:sapphire laser 50 a may include a pumping source 59 afor introducing a pumping beam into the laser resonator.

The first pumping mirror 54 a may be configured to transmit the pumpingbeam from the outside of the laser resonator with high transmittance andreflect the laser beam inside the laser resonator with high reflectance.The Ti:sapphire crystal 55 a may serve as a laser medium that undergoesstimulated emission with the pumping beam. The two prisms 57 a and 58 amay selectively transmit a laser beam at a predetermined wavelength. Theoutput coupler 52 a may transmit a part of the laser beam amplified inthe laser resonator and output the amplified laser beam from the laserresonator, and reflect the remaining part of the laser beam back intothe laser resonator. The semiconductor saturable absorber mirror 51 amay have a reflective layer and a saturable absorber layer laminatedthereon. A part of an incident laser beam of low beam intensity may beabsorbed by the saturable absorber layer, and another part of theincident laser beam of high beam intensity may be transmitted throughthe saturable absorber layer and reflected by the reflective layer. Withthis, the pulse duration of the incident laser beam may be shortened.

A semiconductor pumped Nd:YVO₄ laser may, for example, be used as thepumping source 59 a. The second harmonic wave from the pumping source 59a may be introduced into the laser resonator through the first pumpingmirror 54 a. The position of the semiconductor saturable absorber mirror51 a may be adjusted so as to adjust the resonator length forpredetermined longitudinal modes. With this mode-locking of theTi:sapphire laser 50 a, a picosecond pulse laser beam may be outputtedthrough the output coupler 52 a. Here, when the pulse energy is small,the pulse laser beam may be amplified by a regenerative amplifier.

According to the fifth embodiment, a target material may be irradiatedwith a picosecond pulse laser beam or a pulse laser beam having ashorter pulse duration. When the target material is irradiated with ashort pulse laser beam, thermal diffusion at the irradiation portion maybe made extremely small. Accordingly, energy that may be diffused can beused for the ablation effect. As a result, according to the fifthembodiment, compared to the nanosecond pulse laser beam, a droplet maybe diffused with smaller pulse energy.

8. Sixth Embodiment

FIG. 16 schematically illustrates an exemplary configuration of a fiberlaser configured to output the pre-pulse laser beam in an EUV lightgeneration system according to a sixth embodiment. A fiber laser 50 b ofthe sixth embodiment may be provided outside the chamber 1 as a driverlaser for outputting the pre-pulse laser beam in any one of the firstthrough fourth embodiments.

The fiber laser 50 b may include a laser resonator formed by ahigh-reflection mirror 51 b and a semiconductor saturable absorbermirror 52 b. A grating pair 53 b, a first polarization maintenance fiber54 b, a multiplexer 55 b, a separation element 56 b, a secondpolarization maintenance fiber 57 b, and a focusing optical system 58 bmay be provided in this order from the side of the high-reflectionmirror 51 b in the beam path in the laser resonator. Further, the fiberlaser 50 b may include a pumping source 59 b for introducing a pumpingbeam into the laser resonator.

The multiplexer 55 b may be configured to introduce the pumping beamfrom the pumping source 59 b to the first polarization maintenance fiber54 b and may transmit a laser beam traveling back and forth between thefirst polarization maintenance fiber 54 b and the second polarizationmaintenance fiber 57 b. The first polarization maintenance fiber 54 bmay be doped with ytterbium (Yb), and may undergo stimulated emissionwith the pumping beam. The grating pair 53 b may selectively reflect alaser beam at a predetermined wavelength. The semiconductor saturableabsorber mirror 52 b may be similar in configuration and function to thesemiconductor saturable absorber mirror 51 b in the fifth embodiment.The separation element 56 b may separate a part of the laser beamamplified in the laser resonator and output the separated laser beamfrom the laser resonator and return the remaining part of the laser beamback into the laser resonator. This configuration may lead tomode-locking of the fiber laser 50 b. When the pumping beam from thepumping source 59 b is introduced into the multiplexer 55 b through anoptical fiber, a picosecond pulse laser beam may be outputted throughthe separation element 56 b.

According to the sixth embodiment, in addition to the effect similar tothat of the fifth embodiment, the target material may be irradiated withthe pre-pulse laser beam with high precision since the pre-pulse laserbeam is introduced through an optical fiber. Further, generally, in afiber laser, the M² value that expresses deviation from an idealGaussian distribution of the laser beam intensity distribution isapproximately 1.2. The M² value being closer to 1 means that thefocusing performance is high. Accordingly, when a fiber laser is used, asmall target may be irradiated with a pre-pulse laser beam with highprecision.

The shorter the wavelength of a laser beam, the higher the absorptivityof the laser beam by tin. Accordingly, when the priority is placed onthe absorptivity of the laser beam by tin, a laser beam at a shorterwavelength may be advantageous. For example, compared to the fundamentalharmonic wave outputted from an Nd:YAG laser apparatus at a wavelengthof 1064 nm, the absorptivity may increase with the second harmonic wave(a wavelength of 532 nm), further with the third harmonic wave (awavelength of 355 nm), and even further with the fourth harmonic wave (awavelength of 266 nm).

Here, an example where a picosecond pulse laser beam is used is shown.However, similar effects can be obtained even with a femtosecond pulselaser beam. Further, a droplet can be diffused even with a nanosecondpulse laser beam. For example, a fiber laser with such specifications asa pulse duration of approximately 15 ns, a repetition rate of 100 kHz,pulse energy of 1.5 mJ, a wavelength of 1.03 μm, and the M² value ofbelow 1.5 may be used as a pre-pulse laser apparatus.

9. Irradiation Conditions of Laser Beam

FIGS. 17A and 17B are tables showing irradiation conditions of the laserbeams in the EUV light generation system in any one of the embodiments.When irradiation pulse energy is E (J), a pulse duration is Tp (s), andan irradiation spot size is Dm (m), beam intensity W (W/m²) of the laserbeam may be expressed in Expression (5) below.W=E/(Tp(Em/2)²π)  (5)

FIG. 17A shows four examples (case 1 through case 4) of irradiationconditions of the pre-pulse laser beam. In the case 1, the diameter of amolten tin droplet is 60 μm. The irradiation conditions for diffusingsuch a droplet and generating a desired diffused target may be asfollows. For example, when the irradiation spot size Dm is 100 μm, thebeam intensity W of the laser beam at 1.6×10⁹ W/cm² is required. In thatcase, the irradiation pulse energy E may be set to 1.9 mJ, and the pulseduration Tp may be set to 15 ns. With such a pre-pulse laser beam, adiffused target as shown in FIG. 3B may be generated.

In the case 2 shown in FIG. 17A, the diameter of a molten tin droplet is10 μm (i.e., a mass-limited target). The irradiation conditions fordiffusing such a droplet and generating a desired diffused target may beas follows. For example, when the irradiation spot size Dm is 30 μm, thebeam intensity W of the laser beam at 1.6×10⁹ W/cm² is required. In thatcase, the irradiation pulse energy E may be set to 0.17 mJ, and thepulse duration Tp may be set to 15 ns. With such a pre-pulse laser beam,a diffused target as shown in FIG. 7B may be generated.

In the cases 3 and 4 shown in FIG. 17A, the laser apparatus as shown inFIG. 15 or 16 is used for outputting the pre-pulse laser beam. Further,in the cases 3 and 4, the droplet is a mass-limited target, and the beamintensity W of the laser beam at 1×10¹⁰ W/cm² is required.

FIG. 17B shows four examples (case 1 through case 4) of irradiationconditions of the main pulse laser beam. In the case 1, the diffusiondiameter of a diffused target is 250 μm. Irradiation conditions forturning such a diffused target into plasma may be as follows. Forexample, when the irradiation spot size Dm is 250 μm, the beam intensityW of the laser beam at 1.0×10¹⁰ W/cm² is required. In that case, theirradiation pulse energy E may be set to 100 mJ, and the pulse durationTp may be set to 20 ns. Accordingly, energy required to turn thediffused target into plasma may be supplied to the diffused target.

In the case 2 shown in FIG. 17B, the diffusion diameter of the diffusedtarget, the irradiation spot size Dm, and the beam intensity W of thelaser beam are the same as in the case 1 shown in FIG. 17B. In thatcase, the irradiation pulse energy E may be set to 150 mJ, and the pulseduration Tp may be set to 30 ns. With this, energy required to turn thediffused target into plasma may be supplied to the diffused target.

In the case 3 shown in FIG. 17B, the diffusion diameter of a diffusedtarget is 300 μm. Irradiation conditions for turning such a diffusedtarget into plasma may be as follows. For example, when the irradiationspot size Dm is 300 μm, the beam intensity W of the laser beam at1.1×10¹⁰ W/cm² is required. In that case, the irradiation pulse energy Emay be set to 200 mJ, and the pulse duration Tp may be set to 25 ns.Thus, energy required to turn the diffused target into plasma may besupplied to the diffused target.

In the case 4 shown in FIG. 17B, the diffusion diameter of a diffusedtarget is 200 μm. Irradiation conditions for turning such a diffusedtarget into plasma may be as follows. For example, when the irradiationspot size Dm is 200 μm, the beam intensity W of the laser beam at1.2×10¹⁰ W/cm² is required. In that case, the irradiation pulse energy Emay be set to 200 mJ, and the pulse duration Tp may be set to 50 ns.With this, energy required to turn the diffused target into plasma maybe supplied to the diffused target.

As described above, the beam intensity of the pre-pulse laser beam andthe main pulse laser beam may be set by setting the irradiation pulseenergy E and the pulse duration Tp of the laser beam.

10. Seventh Embodiment

FIG. 18 schematically illustrates an exemplary configuration of an EUVlight generation system according to a seventh embodiment. In the EUVlight generation system according to the seventh embodiment, thepolarization state of the pre-pulse laser beam from a fiber laserapparatus 31 may be controlled by a polarization converter 20. Thepolarization converter 20 may be configured to change the polarizationstate of the pre-pulse laser beam into a state other than the linearpolarization. The polarization converter 20 may be provided at apredetermined position in a beam path between the driver laser and theplasma generation region PS. In this disclosure, a polarization retarderis also included in the polarization converter.

In the seventh embodiment, the fiber laser apparatus 31 may include afiber laser controller 31 a and the fiber laser 50 b described withreference to FIG. 16 (the sixth embodiment). A CO₂ pulse laser apparatus32 may include a CO₂ laser controller 32 a, the master oscillator 3 d,the preamplifier 3 h, the main amplifier 3 j, and the relay opticalsystems 3 g, 3 i, and 3 k as described with reference to FIG. 9 (thefirst embodiment).

The EUV light generation controller 7 may output a fiber laser beamintensity setting signal to the fiber laser controller 31 a. Further,the EUV light generation controller 7 may output a CO₂ laser beamintensity setting signal to the CO₂ laser controller 32 a.

The trigger controller 17 may output a fiber laser oscillation triggersignal to the fiber laser 50 b. Further, the trigger controller 17 mayoutput a CO₂ laser oscillation trigger signal to the master oscillator 3d.

The fiber laser 50 b may be configured to output a pre-pulse laser beamat a first wavelength based on the fiber laser oscillation triggersignal. The fiber laser controller 31 a may be configured to control theoutput intensity of the fiber laser 50 b based on the fiber laser beamintensity setting signal. The pre-pulse laser beam from the fiber laser50 b may be expanded in diameter by the beam expander 4. Thereafter, thepolarization state of the pre-pulse laser beam may be changed by thepolarization converter 20, and then the pre-pulse laser beam may beincident on the beam combiner 15 c.

The master oscillator 3 d may be configured to output a seed beam at asecond wavelength based on the CO₂ laser oscillation trigger signal. TheCO₂ laser controller 32 a may be configured to control the outputintensity of the preamplifier 3 h and the main amplifier 3 j based onthe CO₂ laser beam intensity setting signal. The seed beam from themaster oscillator 3 d may be amplified by the preamplifier 3 h and themain amplifier 3 j to desired beam intensity.

In the seventh embodiment, the fiber laser 50 b is used to output thepre-pulse laser beam. This disclosure, however, is not limited thereto.For example, a YAG laser or a Ti:sapphire laser may be used to outputthe pre-pulse laser beam. Alternatively, in a configuration wheretwo-stage irradiation with the first and second pre-pulse laser beams isemployed, the first pre-pulse laser beam may be outputted from a fiberlaser apparatus capable of achieving a small spot, and the secondpre-pulse laser beam may be outputted from a YAG laser apparatus or aTi:sapphire laser apparatus capable of outputting an ultrashort pulselaser beam. Then, the main pulse laser beam may be outputted from a CO₂laser apparatus capable of achieving high power laser beam. That is, adesired number of pre-pulse laser beams may be outputted from aplurality of separate laser apparatuses. Further, in accordance with thestate of the diffused target at the time of being irradiated with thesecond pre-pulse laser beam, the diffused target may be irradiated witha plurality of pre-pulse laser beams respectively at differentwavelengths, and with difference spot sizes, energy, and pulsedurations.

10.1 Overview of Polarization Control

FIGS. 19A and 20A are conceptual diagrams showing a droplet beingirradiated with a linearly-polarized pre-pulse laser beam. FIGS. 19B and20B show the simulation result of a droplet being irradiated with alinearly-polarized pre-pulse laser beam. In FIGS. 19A and 19B, thedroplet is viewed in a direction (X-direction) perpendicular to thepolarization direction of the pre-pulse laser beam. In FIGS. 20A and20B, the droplet is viewed in a direction of the beam axis (Z-direction)of the pre-pulse laser beam.

With reference to FIGS. 19A and 20A, a case where a droplet isirradiated with a linearly-polarized pre-pulse laser beam will bediscussed. In this case, the droplet may be diffused, and as shown inFIGS. 19B and 20B, a diffused target may be generated. The simulationresult reveals that the diffused target is diffused further in adirection (X-direction) perpendicular to the polarization direction(Y-direction) of the pre-pulse laser beam. When the diffused targetdiffused as such is irradiated with the main pulse laser beam travelingalong substantially the same path as the pre-pulse laser beam, as shownin FIGS. 19B and 20B, the shape of the diffused target may differlargely from the cross-sectional shape of the main pulse laser beam.Accordingly, a large portion of the main pulse laser beam may not beused to generate plasma.

Here, a reason why the diffused target is diffused largely in adirection (X-direction) perpendicular to the polarization direction ofthe linearly-polarized pre-pulse laser beam will be considered. FIG. 21is a graph showing absorptivity of a P-polarization component and anS-polarization component of a laser beam incident on the surface of amolten tin droplet. In the case shown in FIG. 21, the wavelength of thelaser beam is 1.06 μm. As shown in the graph, the absorptivity of thelaser beam may depend on the angle of incidence and the polarizationstate of the laser beam.

The absorptivity of the P-polarization component of an incident laserbeam is at the highest when the angle of incidence of the laser beam is80 to 85 degrees, and gradually decreases as the angle of incidenceshifts from that angle range. On the other hand, the absorptivity of theS-polarization component is substantially the same as that of theP-polarization component when the laser beam is incident on the surfaceof the molten tin droplet at substantially 0 degree (i.e., substantiallynormal incidence), and decreases as the angle of incidence increases.For example, when the angle of incidence is equal to or greater than 80degrees, the absorptivity of the S-polarization component approximatesto 0%.

Based on such absorptivity properties, it is speculated that energy ofthe laser beam is absorbed the most where a linearly-polarized laserbeam is incident on the surface of the droplet as the P-polarizationcomponent at a degree within a range of 80 to 85 degrees. Portions ofthe droplet where the laser beam is incident thereon as theP-polarization component at an angle within the above range are areastoward the edges of the irradiation surface in the Y-direction(hereinafter, referred to as “laser ablation region”). That is, theabsorptivity of the laser beam is high in these areas, and strong laserablation may occur. As a result of the reaction of the laser ablation inthe laser ablation regions, a shock wave or sonic wave may propagatetoward the inside of the droplet from the laser ablation regions. Thisshock wave or sonic wave may propagate toward the edges of the dropletin the X-direction as shown in FIG. 20A, and the droplet may be diffusedin the X-direction as shown in FIG. 20B.

Accordingly, in the seventh embodiment, the polarization state of thepre-pulse laser beam may be changed into a polarization state other thanthe linear polarization using the polarization converter 20. Further, bycontrolling the spot size of the pre-pulse laser beam to be equal to orgreater than the diameter (e.g., 40 μm) of the droplet, the entireirradiation surface of the droplet may be irradiated with the pre-pulselaser beam. With this, the droplet may be diffused symmetrically aboutthe beam axis of the pre-pulse laser beam, and the diffused target maybe irradiated with the main pulse laser beam efficiently.

The polarization converter 20 may be configured to change the pre-pulselaser beam into a substantially circularly-polarized laser beam, asubstantially unpolarized laser beam, a substantially radially-polarizedlaser beam, a substantially azimuthally-polarized laser beam, and soforth.

10.2 Examples of Polarization Control

FIGS. 22A and 22B show a droplet being irradiated with acircularly-polarized pre-pulse laser beam. FIGS. 22C and 22D show adiffused target generated when the droplet is irradiated with thepre-pulse laser beam being irradiated with a main pulse laser beam.FIGS. 22E and 22F schematically show plasma generated when the diffusedtarget is irradiated with the main pulse laser beam.

In a circularly-polarized laser beam, the polarization vector draws acircle on a plane (X-Y plane) perpendicular to the beam axis of thelaser beam. Further, the polarization state of the pre-pulse laser beamis circular at any position along the X-Y plane (see FIGS. 22A and 22B).In the circularly-polarized laser beam, the ratio of an X-directionpolarization component and a Y-direction polarization component issubstantially 1:1. When a droplet is irradiated with thecircularly-polarized pre-pulse laser beam, the distribution ofabsorptivity of the pre-pulse laser beam in the surface of the dropletmay be symmetrical about the center axis of the droplet in theirradiation direction of the laser beam. As a result, the diffusionstate of the droplet may be symmetrical about the center axis of thedroplet, and the shape of the diffused target may become disc-like (seeFIGS. 22C and 22D). This allows the shape of the diffused target tosubstantially coincide with the cross-sectional shape of the main pulselaser beam so that the main pulse laser beam may be absorbed efficientlyby the diffused target.

FIGS. 23A and 23B show a droplet being irradiated with an unpolarizedpre-pulse laser beam. FIGS. 23C and 23D show a diffused target generatedwhen the droplet is irradiated with the pre-pulse laser beam beingirradiated with a main pulse laser beam. FIGS. 23E and 23F schematicallyshow plasma generated when the diffused target is irradiated with themain pulse laser beam.

The pre-pulse laser beam shown in FIG. 23B is substantially unpolarized.In such an unpolarized laser beam, the ratio of the X-directionpolarization component and the Y-direction polarization component issubstantially 1:1. When a droplet is irradiated with the unpolarizedpre-pulse laser beam, the distribution of absorptivity of the pre-pulselaser beam in the surface of the droplet may be symmetrical about thecenter axis of the droplet in the irradiation direction of the laserbeam. As a result, the diffusion state of the droplet may be symmetricalabout the center axis of the droplet, and the shape of the diffusedtarget may, for example, become disc-like. Accordingly, the main pulselaser beam may be absorbed by the diffused target efficiently.

FIGS. 24A and 24B show a droplet being irradiated with aradially-polarized pre-pulse laser beam. FIGS. 24C and 24D show adiffused target generated when the droplet is irradiated with thepre-pulse laser beam being irradiated with a main pulse laser beam.FIGS. 24E and 24F schematically show plasma generated when the diffusedtarget is irradiated with the main pulse laser beam.

When a droplet is irradiated with the radially-polarized pre-pulse laserbeam, the distribution of absorptivity of the pre-pulse laser beam inthe surface of the droplet may be symmetrical about the beam axis of thepre-pulse laser beam. Here, the beam axis of the pre-pulse laser beammay coincide with the center axis of the droplet. As a result, thediffusion state of the droplet may be symmetrical about the beam axis ofthe pre-pulse laser beam, and the shape of the diffused target may, forexample, become disc-like. Accordingly, the main pulse laser beam may beabsorbed by the diffused target efficiently.

Further, when the spot size of the pre-pulse laser beam is controlled tobe equal to or greater than the diameter (e.g., 40 μm) of the droplet,the entire irradiation surface of the droplet may be irradiated with thepre-pulse laser beam incident thereon mostly as the P-polarizationcomponent. Accordingly, the absorptivity of the pre-pulse laser beam maybe increased, and the energy required to generate a desired diffusedtarget may be kept small.

FIGS. 25A and 25B show a droplet being irradiated with anazimuthally-polarized pre-pulse laser beam. FIGS. 25C and 25D show adiffused target generated when the droplet is irradiated with thepre-pulse laser beam being irradiated with a main pulse laser beam.FIGS. 25E and 25F schematically show plasma generated when the diffusedtarget is irradiated with the main pulse laser beam.

When a droplet is irradiated with the azimuthally-polarized pre-pulselaser beam, the distribution of absorptivity of the pre-pulse laser beamin the surface of the droplet may be symmetrical about the beam axis ofthe pre-pulse laser beam. Here, the beam axis of the pre-pulse laserbeam may coincide with the center axis of the droplet. As a result, thediffusion state of the droplet may be symmetrical about the beam axis ofthe pre-pulse laser beam, and the shape of the diffused target may, forexample, become disc-like. Accordingly, the main pulse laser beam may beabsorbed by the diffused target efficiently.

In the seventh embodiment, the distribution of the absorptivity of thepre-pulse laser beam in the surface of the droplet is made symmetricalabout the center axis of the droplet and/or the beam axis of thepre-pulse laser beam by controlling the polarization state of thepre-pulse laser beam. However, this disclosure is not limited thereto.The distribution of the absorptivity of the pre-pulse laser beam in thesurface of the droplet need not be perfectly symmetrical about the beamaxis, but may be substantially symmetrical. Accordingly, thepolarization state of the pre-pulse laser beam may, for example, beelliptical as well.

FIG. 26A schematically illustrates an exemplary configuration of adevice for measuring the degree of linear polarization. The device mayinclude a polarization prism and a beam intensity detector. FIG. 26Bshows the relationship between the rotation angle of the polarizationprism and the detection result of the beam intensity detector.

As shown in FIG. 26A, a linearly-polarized pre-pulse laser beam from thefiber laser 50 b may be changed into an elliptically-polarized laserbeam by the polarization converter 20. This elliptically-polarized laserbeam may be focused by a focusing optical system 41 and made to beincident on the polarization prism 42. The beam intensity of the laserbeam outputted from the polarization prism 42 may be detected by thebeam intensity detector 43. The polarization prism 42 may be formed bybonding two refractive crystals such as calcite. The polarization prism42 may be used to extract a laser beam of a predetermined polarizationdirection as an output laser beam from an input beam in accordance withthe orientation of the bonding surface of the prism. As the polarizationprism 42 is rotated about the beam axis of the pre-pulse laser beam, thepolarization prism 42 may transmit a laser beam polarized in a directioncorresponding to the rotation angle. In the description below, it isassumed that the polarization prism 42 may be an ideal prism having asufficiently high extinction factor.

As shown in FIG. 26B, the beam intensity of the output beam from thepolarization prism 42 may change periodically as the polarization prism42 is rotated by 180 degrees. Here, as shown in Expression (6), thedegree of linear polarization Po may be obtained from a maximum valueImax and a minimum value Imin of the beam intensity.Po=(Imax−Imin)/(Imax+Imin)×100(%)  (6)

The degree of linear polarization Po measured by the device shown inFIG. 26A may be substantially 0% for a laser beam of a polarizationstate that is substantially symmetrical about the beam axis (e.g.,circularly-polarized laser beam, unpolarized laser beam,radially-polarized laser beam, azimuthally-polarized laser beam). On theother hand, the degree of linear polarization Po may be substantially100% for a linearly-polarized laser beam. Here, when the degree oflinear polarization Po is in the following ranges, the diffused targetmay be formed in a desired shape (e.g., disc-shape).

-   -   0%≦Po<30%    -   0%≦Po<20%    -   0%≦Po<10%        These ranges may be adjusted with the extinction factor of the        actually-used polarization prism 42 taken into consideration.        10.3 Examples of Polarization Converter

FIG. 27 shows a first example of a polarization converter in the seventhembodiment. In FIG. 27, a quarter-wave plate 21 for converting alinearly-polarized laser beam into a circularly-polarized laser beam maybe used as the polarization converter.

The transmissive quarter-wave plate 21 may be a refractive crystal thatprovides a phase difference of π/2 between a polarization componentparallel to the optic axis of the crystal and a polarization componentperpendicular to the optic axis of the crystal. As shown in FIG. 27, alinearly-polarized laser beam may be converted into acircularly-polarized laser beam when the linearly-polarized laser beamis incident on the quarter-wave plate 21 such that the polarizationdirection thereof is inclined by 45 degrees with respect to the opticaxis of the quarter-wave plate 21. When the polarization direction ofthe linearly-polarized laser beam is inclined by 45 degrees in the otherdirection, the rotation direction of the circular polarization isreversed. This disclosure is not limited to the transmissivequarter-wave plate 21, and a reflective quarter-wave plate may be usedas well.

FIGS. 28A through 28C show a second example of a polarization converterin the seventh embodiment. FIG. 28A is a front view of the polarizationconverter, FIG. 28B is an enlarged fragmentary sectional view of thepolarization converter taken along a radial direction plane, and FIG.28C shows one mode for the use of the polarization converter. In FIGS.28A through 28C, a random phase plate 22 for converting alinearly-polarized laser beam into an unpolarized laser beam may be usedas the polarization control apparatus.

The transmissive random phase plate 22 may be a transmissive opticalelement having a diameter D, on whose input or output surface minutesquare regions each having a length d on each side are formed byrandomly arranged recesses and protrusions. The random phase plate 22may divide an input beam having the diameter D into small square beamseach having the length d on each side. With this configuration, therandom phase plate 22 may provide a phase difference of π between asmall beam transmitted through a protrusion 22 a and a small beamtransmitted through a recess 22 b. The phase difference π may beprovided by setting a step Δt between the protrusion 22 a and the recess22 b as in Expression (7) below, where the wavelength of the incidentlaser beam is λ, and the refractive index of the random phase plate 22is n₁.Δt=λ/2(n ₁−1)  (7)

As shown in FIG. 28C, the transmissive random phase plate 22 may, forexample, be provided between the pre-pulse laser apparatus and thefocusing optical system 15. A linearly-polarized laser beam may beincident on the random phase plate 22, and the laser beam transmittedthrough the random phase plate 22 may become unpolarized. Laser beamspolarized in directions perpendicular to each other do not interfere.Accordingly, when this laser beam is focused by the focusing opticalsystem 15, the cross-sectional beam intensity distribution at the focusmay not be Gaussian but may be closer to the top-hat distribution. Whena droplet is irradiated with such a pre-pulse laser beam, the dropletmay be diffused substantially symmetrically about the center axis of thedroplet. Accordingly, the diffused target may become disc-shaped, andthe main pulse laser beam may be absorbed by the diffused targetefficiently.

This disclosure is not limited to the transmissive random phase plate22, and a reflective random phase plate may be used instead. Further,the protrusion 22 a and the recess 22 b may be in any other polygonalshapes, such as a hexagonal shape, a triangular shape.

FIGS. 29A and 29B show a third example of a polarization converter inthe seventh embodiment. FIG. 29A is a perspective view of thepolarization converter, and FIG. 29B is a front view of the polarizationconverter. FIGS. 29A and 29B show an n-divided wave plate 23 forconverting a linearly-polarized laser beam into a radially-polarizedlaser beam.

The n-divided wave plate 23 may be a transmissive optical element inwhich n triangular half-wave plates 231, 232, . . . , 23 n are arrangedsymmetrically about the beam axis of the laser beam. Each of thehalf-wave plates 231, 232, . . . , 23 n may be a refractive crystal thatprovides a phase difference of π between a polarization componentparallel to the optic axis of the crystal and a polarization componentperpendicular to the optic axis of the crystal. When alinearly-polarized laser beam is incident on such a half-wave plateperpendicularly such that the polarization direction is inclined by anangle θ with respect to the optic axis of the half-wave plate, the laserbeam may be outputted from the half-wave plate with its polarizationdirection being rotated by 2θ.

For example, the half-wave plate 231 and the half-wave plate 233 may bearranged so that their respective optic axes make an angle of 45degrees. Then, the polarization direction of the linearly-polarizedlaser beam transmitted through the half-wave plate 231 and thepolarization direction of the linearly-polarized laser beam transmittedthrough the half-wave plate 233 may differ by 90 degrees. In this way,the polarization direction of the incident laser beam may be changed inaccordance with an angle formed by the optic axis of the half-wave plateand the polarization direction of the incident laser beam. With this,the polarization directions of the laser beams transmitted through therespective half-wave plates may be changed to predetermined polarizationdirections. As a result, the re-divided wave plate 23 may convert alinearly-polarized laser beam into a radially-polarized laser beam.Further, by changing the arrangement of the half-wave plates in then-divided wave plate 23, a linearly-polarized laser beam may beconverted into an azimuthally-polarized laser beam as well.

FIG. 30 shows a fourth example of a polarization converter in theseventh embodiment. FIG. 30 shows a phase compensator 24 a, apolarization rotator 24 b, and a theta cell 24 c for converting alinearly-polarized laser beam into a radially-polarized laser beam.

The theta cell 24 c may be an optical element into which a twistednematic (TN) liquid crystal is injected, and the liquid crystalmolecules are arranged so as to be twisted from the input side towardthe output side. A linearly-polarized laser beam incident on the thetacell 24 c may be rotated along the twist of the alignment of the liquidcrystal molecules, and a laser beam linearly-polarized in a directioninclined with respect to the polarization direction of the input beammay be outputted from the theta cell 24 c. Accordingly, by setting thetwisted angle of the alignment of the liquid crystal molecules in thetheta cell 24 c so as to differ in accordance with the azimuth angledirection, the theta cell 24 c may convert a linearly-polarized inputbeam into a radially-polarized output beam.

However, when a linearly-polarized laser beam is converted into aradially-polarized laser beam only with the theta cell 24 c, the beamintensity may be decreased at a boundary between an upper half and alower half of the laser beam outputted from the theta cell 24 c.Accordingly, a phase of the upper half of the laser beam may be shiftedby π by the phase compensator 24 a prior to the laser beam beingincident on the theta cell 24 c. In FIG. 30, the arrows indicate thatthe phases of the input beam are opposite between the upper and lowerhalves of the laser beam. The upper half of the phase compensator 24 amay include a TN liquid crystal in which the alignment of the liquidcrystal molecules is twisted by 180 degrees from the input side towardthe output side. In this way, when a linearly-polarized laser beam inwhich the phases of the upper and lower halves are opposite is made tobe incident on the theta cell 24 c, laser beams of the same phase may beoutputted around the boundary between the upper and lower halves of theoutput laser beam. With this, the beam intensity may be prevented frombeing decreased at the boundary between the upper and lower halves ofthe laser beam outputted from the theta cell 24 c.

The polarization rotator 24 b may be configured to rotate thepolarization direction of the linearly-polarized input beam by 90degrees. When a laser beam of which the polarization direction isrotated by 90 degrees is made to be incident on the theta cell 24 c, thetheta cell 24 c may convert the linearly-polarized laser beam into anazimuthally-polarized laser beam. The polarization rotator 24 b may beformed of a TN liquid crystal in which the alignment of the liquidcrystal molecules is twisted by 90 degrees from the input side towardthe output side. In this case, by controlling the DC voltage applied tothe polarization rotator 24 b so as to switch between a state where thealignment of the liquid crystal molecules are twisted and a state wherethe alignment is not twisted, switching between a radially-polarizedoutput beam and an azimuthally-polarized output beam may be achieved.

In this way, the conversion of the polarization state may be achievedrelatively freely by using the phase compensator 24 a, the polarizationrotator 24 b, and the theta cell 24 c. Further, as described withreference to FIGS. 27 through 29B, when the polarization direction is tobe changed using a wave plate (phase plate), the wavelength of the laserbeam of which the polarization direction is changed may differ dependingon the thickness of the wave plate. However, as described with referenceto FIG. 30, when the theta cell 24 c is used, the polarization directionof an input beam of a relatively broad bandwidth may be changed.Accordingly, using the theta cell 24 c may make it possible to changethe polarization direction even when the bandwidth of the pre-pulselaser beam is broad.

11. Eighth Embodiment

FIG. 31 schematically illustrates the configuration of an EUV lightgeneration system according to an eighth embodiment. In the EUV lightgeneration system according to the eighth embodiment, the polarizationstate of a pre-pulse laser beam from the fiber laser apparatus 31 may becontrolled by the polarization converter 20, and this pre-pulse laserbeam may be guided into the chamber 1 along a beam path that isdifferent from that of the main pulse laser beam.

12. Ninth Embodiment

FIGS. 32A through 32C schematically illustrates an exemplaryconfiguration of a laser apparatus configured to output a pre-pulselaser beam in an EUV light generation system according to a ninthembodiment. A laser apparatus 60 a of the ninth embodiment may beprovided outside the chamber 1 (see, e.g., FIG. 1) as a driver laser foroutputting a pre-pulse laser beam in any one of the first through fourthembodiments.

As shown in FIG. 32A, the laser apparatus 60 a may include a laserresonator that includes a reflective polarization converter 61 a and afront mirror 62. A laser medium 63 may be provided in the laserresonator. Stimulated emission light may be generated from the lasermedium 63 with a pumping beam from a pumping source (not shown). Thestimulated emission light may travel back and forth between thepolarization converter 61 a and the front mirror 62 and be amplified bythe laser medium 63. Thereafter, an amplified laser beam may beoutputted from the laser apparatus 60 a.

The polarization converter 61 a may be configured to reflect with highreflectance a laser beam of a predetermined polarization direction inaccordance with the input position on the polarization converter 61 a.In accordance with the reflective properties of the polarizationconverter 61 a, a radially-polarized laser beam shown in FIG. 32B or anazimuthally-polarized laser beam shown in FIG. 32C may be amplified inthe laser resonator. A part of the amplified laser beam may betransmitted through the front mirror 62 and outputted as the pre-pulselaser beam.

According to the ninth embodiment, a polarization converter may be usedas a part of the resonator of the driver laser. With this, apolarization converter need not be provided in a beam path between thedriver laser and the plasma generation region PS as in the seventhembodiment.

FIGS. 33A through 33C schematically illustrates the exemplaryconfiguration of a laser apparatus configured to output a pre-pulselaser beam in an EUV light generation system according to a modificationof the ninth embodiment. A laser apparatus 60 b of this modification mayinclude a laser resonator that includes a rear mirror 61 and areflective polarization converter 62 a. In accordance with thereflective properties of the polarization converter 62 a, aradially-polarized laser beam shown in FIG. 33B or anazimuthally-polarized laser beam shown in FIG. 33C may be amplified inthe laser resonator. A part of the amplified laser beam may betransmitted through the polarization converter 62 a and outputted as thepre-pulse laser beam.

FIGS. 34A and 34B show an example of a polarization converter in theninth embodiment. FIG. 34A is a perspective view of the polarizer, andFIG. 34B is an enlarged fragmentary sectional view of a diffractiongrating portion of the polarization converter, taken along the radialdirection plane. As shown in FIG. 34A, the reflective polarizationconverter 61 a may be a mirror on which a concentric circulardiffraction grating 611 is formed. Further, as shown in FIG. 34B, in thepolarization converter 61 a, a multilayer film 612 may be formed on aglass substrate 613, and the diffraction grating 611 may be formed onthe multilayer film 612.

When an azimuthally-polarized laser beam is incident on the polarizationconverter 61 a configured as such (here, the polarization direction issubstantially parallel to the direction of the grooves in thediffraction grating 611), the azimuthally-polarized laser beam may betransmitted through the diffraction grating 611 and propagated to themultilayer film 612. On the other hand, when a radially-polarized laserbeam is incident on the polarization converter 61 a configured as such(here, the polarization direction is substantially perpendicular to thedirection of the grooves in the diffraction grating 611), theradially-polarized laser beam may not be transmitted through thediffraction grating 611 and may be reflected thereby. In the ninthembodiment (see FIGS. 32A through 32C), using the polarization converter61 a configured as such in the laser resonator may make it possible tooutput a radially-polarized laser beam.

Here, when the grooves in the diffraction grating 611 are formedradially, the polarization converter 61 a may reflect anazimuthally-polarized laser beam with high reflectance. In this case,the azimuthally-polarized laser beam may be outputted. Further, formingthe diffraction grating 611 on the polarization converter 62 a of themodification (see FIGS. 33A through 33C) of the ninth embodiment maymake it possible to output a radially-polarized laser beam or anazimuthally-polarized laser beam.

13. Control of Fluence

FIG. 35 is a graph on which the obtained conversion efficiency (CE) inaccordance with a fluence (energy per unit area of a beam cross-sectionat its focus) of a pre-pulse laser beam is plotted.

The measuring conditions are as follows. A molten tin droplet having adiameter of 20 μm is used as a target material. A laser beam with apulse duration of 5 ns to 15 ns outputted from a YAG pulse laserapparatus is used as a pre-pulse laser beam. A laser beam with a pulseduration of 20 ns outputted from a CO₂ pulse laser apparatus is used asa main pulse laser beam. The beam intensity of the main pulse laser beamis 6.0×10⁹ W/cm², and the delay time for the irradiation with the mainpulse laser beam is 1.5 μs after the irradiation with the pre-pulselaser beam.

The horizontal axis of the graph shown in FIG. 35 shows a value in whichthe irradiation conditions of the pre-pulse laser beam (pulse duration,energy, spot size) are converted into a fluence. Further, the verticalaxis shows the CE in the case where the diffused target generated inaccordance with the irradiation conditions of the pre-pulse laser beamis irradiated with the above main pulse laser beam.

The measurement results shown in FIG. 35 reveal that increasing thefluence of the pre-pulse laser beam may improve the CE (approximately3%). That is, at least in a range where the pulse duration of thepre-pulse laser beam is 5 ns to 15 ns, there is a correlation betweenthe fluence and the CE.

Accordingly, in the above embodiments, the EUV light generationcontroller 7 may be configured to control the fluence, instead of thebeam intensity, of the pre-pulse laser beam. The measurement resultsshown in FIG. 35 reveal that the fluence of the pre-pulse laser beam maybe in the range of 10 mJ/cm² to 600 mJ/cm². In other embodiments, therange may be 30 mJ/cm² to 400 mJ/cm². In yet other embodiments, therange may be 150 mJ/cm² to 300 mJ/cm².

From the measurement results where the CE is improved when the fluenceof the pre-pulse laser beam is controlled as above, it is speculatedthat a droplet is diffused in a disc-shape, a dish-shape, or atorus-shape under the above conditions. That is, it is speculated thatwhen a droplet is diffused, the total surface area is increased, theenergy of the main pulse laser beam is absorbed efficiently by thediffused target, and as a result, the CE is improved.

14. Control of Delay Time

FIG. 36 is a graph showing the result of an experiment for generating adiffused target in an EUV light generation system. In this experiment,the EUV light generation system of the eighth embodiment is used. Thepre-pulse laser beam may be converted into a circularly-polarized laserbeam by the polarization converter 20. The horizontal axis in FIG. 36shows a time that has elapsed since a droplet is irradiated with apre-pulse laser beam. The vertical axis shows a diffusion radius of adiffused target generated when the droplet is irradiated with thepre-pulse laser beam. The diffusion radius is a radius of a space wherea particle of a predetermined diameter exists. Changes over time in thediffusion radius after the irradiation with the pre-pulse laser beam areplotted for the droplets respectively having diameters of 12 μm, 20 μm,30 μm, and 40 μm. As seen from FIG. 36, the diffusion radius has lowdependency on the droplet diameter. Further, the changes over time inthe diffusion radius are relatively gradual in 0.3 μs to 3 μs after adroplet is irradiated with a pre-pulse laser beam. It is speculated thatthe variation in the diffusion radius for each droplet is small duringthis time period. Accordingly, if the diffused target is irradiated witha main pulse laser beam during this time period, the variation ingenerated EUV energy may be small between pulses.

FIG. 37 is a graph on which the obtained conversion efficiency (CE) forthe corresponding delay time since a droplet is irradiated with apre-pulse laser beam until a diffused target is irradiated by a mainpulse laser beam is plotted for differing diameters of the droplet.

The measuring conditions are as follows. Molten tin dropletsrespectively having diameters of 12 μm, 20 μm, 30 μm, and 40 μm are usedas the target material. A laser beam with a pulse duration of 5 nsoutputted from a YAG pulse laser apparatus is used as a pre-pulse laserbeam. The fluence of the pre-pulse laser beam is 490 mJ/cm². A laserbeam with a pulse duration of 20 ns outputted from a CO₂ pulse laserapparatus is used as a main pulse laser beam. The beam intensity of themain pulse laser beam is 6.0×10⁹ W/cm².

The measurement results shown in FIG. 37 reveal that the delay time forthe irradiation with the main pulse laser beam may be in a range of 0.5μs to 2.5 μs after the irradiation with the pre-pulse laser beam.However, it is found that the optimum range of the delay time for theirradiation with the main pulse laser beam to obtain a high CE differsdepending on the diameter of the droplet.

When the diameter of the droplet is 12 μm, the delay time for theirradiation with the main pulse laser beam may be in a range of 0.5 μsto 2 μs after the irradiation with the pre-pulse laser beam. In otherembodiments, the range may be 0.6 μs to 1.5 μs. In yet otherembodiments, the range may be 0.7 μs to 1 μs.

When the diameter of the droplet is 20 μm, the delay time for theirradiation with the main pulse laser beam may be in a range of 0.5 μsto 2.5 μs after the irradiation with the pre-pulse laser beam. In otherembodiments, the range may be 1 μs to 2 μs. In yet other embodiments,the range may be 1.3 μs to 1.7 μs.

When the diameter of the droplet is 30 μm, the delay time for theirradiation with the main pulse laser beam may be in a range of 0.5 μsto 4 μs after the irradiation with the pre-pulse laser beam. In otherembodiments, the range may be 1.5 μs to 3.5 μs. In yet otherembodiments, the range may be 2 μs to 3 μs.

When the diameter of the droplet is 40 μm, the delay time for theirradiation with the main pulse laser beam may be in a range of 0.5 μsto 6 μs after the irradiation with the pre-pulse laser beam. In otherembodiments, the range may be 1.5 μs to 5 μs. In yet other embodiments,the range may be 2 μs to 4 μs.

15. Tenth Embodiment 15.1 Configuration

FIG. 38 is a partial sectional view schematically illustrating anexemplary configuration of an EUV light generation system according to atenth embodiment of this disclosure. As shown in FIG. 38, a laser beamfocusing optical system 122, an EUV collector mirror 5, a targetcollector 14, an EUV collector mirror mount 141, plates 142 and 143, abeam dump 144, a beam dump support member 145 may be provided inside thechamber 1.

The plate 142 may be attached to the chamber 1, and the plate 143 may beattached to the plate 142. The EUV collector mirror 5 may be attached tothe plate 142 through the EUV collector mirror mount 141.

The laser beam focusing optical system 122 may include an off-axisparaboloidal mirror 221, a flat mirror 222, and holders 221 a and 222 afor the respective mirrors 221 and 222. The off-axis paraboloidal mirror221 and the flat mirror 222 may be positioned on the plate 143 throughthe respective mirror holders 221 a and 222 a such that a pulse laserbeam reflected by these mirrors 221 and 222 is focused in the plasmageneration region PS.

The beam dump 144 may be fixed in the chamber 1 through the beam dumpsupport member 145 to be positioned on an extension of a beam path of apulse laser beam. The target collector 14 may be provided on anextension of a trajectory of a droplet DL.

A target sensor 104, an EUV light sensor 107, a window 12, and a targetsupply unit 2 may be provided in the chamber 1. A laser apparatus 103, alaser beam travel direction control unit 134, and an EUV light controldevice 105 may be provided outside the chamber 1.

The target sensor 104 may include an imaging function and may detect atleast one of the presence, the trajectory, the position, and the speedof a droplet DL. The EUV light sensor 107 may be configured to detectEUV light generated in the plasma generation region PS to detect anintensity of the EUV light, and output a detection signal to an EUVlight generation controller 151. The target supply unit 2 may beconfigured to continuously output droplets at a predetermined interval,or configured to output a droplet on-demand at a timing in accordancewith a trigger signal received from a droplet controller 152. The laserbeam travel direction control unit 134 may include high-reflectionmirrors 351, 352, and 353, a dichroic mirror 354, and holders 351 a, 352a, 353 a, and 354 a for the respective mirrors 351, 352, 353, and 354.

The EUV light control device 105 may include the EUV light generationcontroller 151, the droplet controller 152, and a delay circuit 153. TheEUV light generation controller 151 may be configured to output controlsignals respectively to the droplet controller 152, the delay circuit153, and the laser apparatus 103.

The laser apparatus 103 may include a pre-pulse laser apparatus 300configured to output a pre-pulse laser beam and a main pulse laserapparatus 390 configured to output a main pulse laser beam. Theaforementioned dichroic mirror 354 may include a coating configured toreflect the pre-pulse laser beam with high reflectance and transmit themain pulse laser beam with high transmittance, and may serve as a beamcombiner.

15.2 Operation

The droplet controller 152 may output a target supply start signal tothe target supply unit 2 to cause the target supply unit 2 to startsupplying the droplets DL toward the plasma generation region PS insidethe chamber 1.

Upon receiving the target supply start signal from the dropletcontroller 152, the target supply unit 2 may start outputting thedroplets DL toward the plasma generation region PS. The dropletcontroller 152 may receive a target detection signal from the targetsensor 104 and output that detection signal to the delay circuit 153.The target sensor 104 may be configured to detect a timing at which adroplet DL passes through a predetermined position prior to reaching theplasma generation region PS. For example, the target sensor 104 mayinclude a laser device (not shown) and an optical sensor. The laserdevice included in the target sensor 104 may be positioned such that acontinuous wave (CW) laser beam from the laser device travels throughthe aforementioned predetermined position. The optical sensor includedin the target sensor 104 may be positioned to detect a ray reflected bythe droplet DL when the droplet DL passes through the aforementionedpredetermined position. When the droplet DL passes through theaforementioned predetermined position, the optical sensor may detect theray reflected by the droplet DL and output a target detection signal.

The delay circuit 153 may output a first timing signal to the pre-pulselaser apparatus 300 so that the droplet DL is irradiated with thepre-pulse laser beam at a timing at which the droplet DL reaches theplasma generation region PS. The first timing signal may be a signal inwhich a first delay time is given to a target detection signal. Thedelay circuit 153 may output a second timing signal to the main pulselaser apparatus 390 such that a diffused target is irradiated with themain pulse laser beam at a timing at which a droplet irradiated with thepre-pulse laser beam is diffused to a predetermined size to form thediffused target. Here, a time from the first timing signal to the secondtiming signal may be a second delay time.

The pre-pulse laser apparatus 300 may be configured to output thepre-pulse laser beam in accordance with the first timing signal from thedelay circuit 153. The main pulse laser apparatus 390 may be configuredto output the main pulse laser beam in accordance with the second timingsignal from the delay circuit 153.

The pre-pulse laser beam from the pre-pulse laser apparatus 300 may bereflected by the high-reflection mirror 353 and the dichroic mirror 354,and enter the laser beam focusing optical system 122 through the window12. The main pulse laser beam from the main pulse laser apparatus 390may be reflected by the high-reflection mirrors 351 and 352, transmittedthrough the dichroic mirror 354, and enter the laser beam focusingoptical system 122 through the window 12.

Each of the pre-pulse laser beam and the main pulse laser beam that haveentered the laser beam focusing optical system 122 may be reflectedsequentially by the off-axis paraboloidal mirror 221 and the flat mirror222, and guided to the plasma generation region PS. The pre-pulse laserbeam may strike the droplet DL, which may be diffused to form a diffusedtarget. This diffused target may then be irradiated with the main pulselaser beam to thereby be turned into plasma.

15.3 Parameters of Pre-Pulse Laser Beam

15.3.1 Relationship Between Pulse Duration and CE

FIG. 39 is a graph showing an example of a relationship between anirradiation condition of a pre-pulse laser beam and a conversionefficiency (CE) in an EUV light generation system. In FIG. 39, a delaytime (a third delay time) (μs) for the main pulse laser beam from thepre-pulse laser beam is plotted along the horizontal axis, and aconversion efficiency (%) from an energy of the main pulse laser beaminto an energy of the EUV light is plotted along the vertical axis. Thethird delay time may be a time from the irradiation of a droplet DL witha pre-pulse laser beam to the irradiation of a diffused target with amain pulse laser beam.

In the graph shown in FIG. 39, seven combination patterns of a pulseduration (the full width at half maximum) and a fluence (energy density)of a pre-pulse laser beam were set, and a measurement was carried out oneach combination pattern. Obtained results are shown in a line graph.Here, a fluence may be a value in which an energy of a pulse laser beamis divided by an area of a portion having a beam intensity equal to orhigher than 1/e² at the spot.

Details on the measuring conditions are as follows. Tin (Sn) was used asthe target material, and tin was molten to produce a droplet having adiameter of 21 μm.

As for the pre-pulse laser apparatus 300, an Nd:YAG laser apparatus wasused to generate a pre-pulse laser beam having a pulse duration of 10 nsand a pulse energy of 0.5 mJ to 2.7 mJ. The wavelength of this pre-pulselaser beam was 1.06 μm. When a pre-pulse laser beam having a pulseduration of 10 μs was to be generated, a mode-locked laser deviceincluding an Nd:YVO₄ crystal was used as a master oscillator, and aregenerative amplifier including an Nd:YAG crystal was used. Thewavelength of this pre-pulse laser beam was 1.06 μm, and the pulseenergy thereof was 0.25 mJ to 2 mJ. The spot size of each of thepre-pulse laser beams was 70 μm.

A CO₂ laser apparatus was used as the main pulse laser apparatus togenerate a main pulse laser beam. The wavelength of the main pulse laserbeam was 10.6 μm, and the pulse energy thereof was 135 mJ to 170 mJ. Thepulse duration of the main pulse laser beam was 15 ns, and the spot sizethereof was 300 μm.

The results are as follows. As shown in FIG. 39, when the pulse durationof the pre-pulse laser beam was 10 ns, a CE never reached 3.5% at themaximum. Further, the CE in this case reached the maximum in eachcombination pattern when the third delay time is equal to or greaterthan 3 μs.

On the other hand, as for a CE when the pulse duration of the pre-pulselaser beam was 10 μs, the maximum value in each combination patternexceeded 3.5%. These maximum values were obtained when the third delaytime was smaller than 3 μs. In particular, the CE of 4.7% was achievedwhen the pulse duration of the pre-pulse laser beam was 10 μs, thefluence was 52 J/cm², and the third delay time was 1.2 μs.

The above-described results reveal that a higher CE may be achieved whenthe pulse duration of the pre-pulse laser beam is in the picosecondrange (e.g., 10 μs) compared to the case where the pulse durationthereof is in the nanosecond range (e.g., 10 ns). Further, an optimalthird delay time for obtaining the highest CE was smaller when the pulseduration of the pre-pulse laser beam was in the picosecond rangecompared to the case where the pulse duration thereof was in thenanosecond range. Accordingly, the EUV light may be generated at ahigher repetition rate when the pulse duration of the pre-pulse laserbeam is in the picosecond range compared to the case where the pulseduration thereof is in the nanosecond range.

Further, based on the results shown in FIG. 39, when the pulse durationof the pre-pulse laser beam is in the picosecond range and the fluenceis 13 J/cm² to 52 J/cm², the third delay time may be set in a rangebetween 0.5 μs and 1.8 μs inclusive. In other embodiments, the thirddelay time may be in a range of between 0.7 μs and 1.6 μs inclusive, andin yet other embodiments, the range may be between 1.0 μs and 1.4 μsinclusive.

15.3.2 Relationship Between Pulse Duration and Fluence, and RelationshipBetween Pulse Duration and Beam Intensity

FIG. 40A is a graph showing an example of a relationship between afluence of a pre-pulse laser beam and a CE in an EUV light generationsystem. In FIG. 40A, a fluence (J/cm²) of a pre-pulse laser beam isplotted along the horizontal axis, and a CE (%) is plotted along thevertical axis. In each of the cases where a pulse duration of thepre-pulse laser beam was set to 10 μs, 10 ns, and 15 ns, a CE wasmeasured for various third delay times, and the CE at the optimal thirddelay time was plotted. Here, the results shown in FIG. 39 were used tofill a part of the data where the pulse duration was 10 μs or 10 ns.Further, in order to generate a pre-pulse laser beam having a pulseduration of 15 ns, a pre-pulse laser apparatus configured similarly tothe one used to generate a pre-pulse laser beam having a pulse durationof 10 ns was used.

In all of the cases where the pulse duration of the pre-pulse laser beamwas 10 μs, 10 ns, and 15 ns, the CE increased with the increase in thefluence of the pre-pulse laser beam, and the CE saturated when thefluence exceeded a predetermined value. Further, the higher CE wasobtained when the pulse duration was 10 μs, compared to the case wherethe pulse duration was 10 ns or 15 ns, and the fluence required toobtain that CE was smaller when the pulse duration was 10 μs. When thepulse duration was 10 μs, if the fluence was increased from 2.6 J/cm² to6.5 J/cm², the CE improved greatly, and if the fluence exceeded 6.5J/cm², the rate of increase in the CE with respect to the increase inthe fluence was small.

FIG. 40B is a graph showing an example of a relationship between a beamintensity of a pre-pulse laser beam and a CE in an EUV light generationsystem. In FIG. 40B, the beam intensity (W/cm²) of the pre-pulse laserbeam is plotted along the horizontal axis, and the CE (%) is plottedalong the vertical axis. The beam intensity was calculated from theresults shown in FIG. 40A. Here, the beam intensity is a value in whichthe fluence of the pre-pulse laser beam is divided by the pulse duration(the full width at half maximum).

In all of the cases where the pulse duration of the pre-pulse laser beamwas 10 μs, 10 ns, and 15 ns, the CE increased with the increase in thebeam intensity of the pre-pulse laser beam. Further, a higher CE wasobtained when the pulse duration was 10 μs, compared to the case wherethe pulse duration was 10 ns or 15 ns. When the pulse duration was 10μs, the CE greatly improved if the beam intensity was increased from2.6×10¹¹ W/cm² to 5.6×10¹¹ W/cm², and an even higher CE was obtainedwhen the beam intensity exceeded 5.6×10¹¹ W/cm².

As described above, when a droplet is irradiated with a pre-pulse laserbeam having a pulse duration in the picosecond range to form a diffusedtarget and the diffused target is irradiated with a main pulse laserbeam, a higher CE may be obtained.

15.3.3 Relationship Between Pulse Duration and State of Diffused Target

FIGS. 41A and 41B show photographs of a diffused target generated when adroplet is irradiated with a pre-pulse laser beam in an EUV lightgeneration system. Each of the photographs shown in FIG. 41A wascaptured with the optimal third delay time in cases where the pulseduration of the pre-pulse laser beam was set to 10 μs with threediffering fluences. That is, as in the description given with referenceto FIG. 39, FIG. 41A shows a diffused target at the third delay times of1.2 μs (fluence of 52 J/cm²), 1.1 μs (fluence of 26 J/cm²), and 1.3 μs(fluence of 13 J/cm²). Each of the photographs shown in FIG. 41B wascaptured with the optimal third delay time in cases where the pulseduration of the pre-pulse laser beam was set to 10 ns with two differingfluences. That is, FIG. 41B shows a diffused target at the third delaytimes of 3 μs (fluence of 70 J/cm²) and 5 μs (fluence of 26 J/cm²). Inboth FIGS. 41A and 41B, the diffused target was captured at an angle of60 degrees and 90 degrees with respect to the beam path of the pre-pulselaser beam. The arrangement of the capturing equipment will be describedlater.

A diameter De of the diffused target was 360 μm to 384 μm when the pulseduration of the pre-pulse laser beam was 10 μs, and the diameter De was325 μm to 380 μm when the pulse duration of the pre-pulse laser beam was10 ns. That is, the diameter De of the diffused target was somewhatlarger than 300 μm, which was the spot size of the main pulse laserbeam. However, the spot size of the main pulse laser beam here is shownas a 1/e² width (a width of a portion having a beam intensity equal toor higher than 1/e² of the peak intensity). Thus, even when the diameterDe of the diffused target is 400 μm, the diffused target may beirradiated with the main pulse laser beam sufficiently.

Further, the diameter De of the diffused target reached 300 μm in ashorter period of time when the pulse duration of the pre-pulse laserbeam was 10 μs, compared to the case where the pulse duration was 10 ns.That is, the diffusion speed of the diffused target was found to befaster when the pulse duration was 10 μs, compared to the case where thepulse duration was 10 ns.

FIG. 42 schematically illustrates an arrangement of equipment used tocapture the photographs shown in FIGS. 41A and 41B. As shown in FIG. 42,cameras C1 and C2 are respectively arranged at 60 degrees and 90 degreesto the beam path of the pre-pulse laser beam, and flash lamps L1 and L2are respectively arranged to oppose the cameras C1 and C2 with a pointwhere a droplet is irradiated located therebetween.

FIGS. 43A and 43B are sectional views schematically illustrating thediffused targets shown respectively in FIGS. 41A and 41B. As shown inFIGS. 41A and 43A, when the pulse duration of the pre-pulse laser beamwas 10 μs, the droplet diffused annularly in the direction in which thepre-pulse laser beam travels, and diffused in a dome shape in theopposite direction. More specifically, the diffused target included afirst portion T1 where the target material diffused in an annular shape,a second portion T2 which is adjacent to the first portion T1 and inwhich the target material diffused in a dome shape, and a third portionT3 surrounded by the first portion T1 and the second portion T2. Thedensity of the target material was higher in the first portion T1 thanin the second portion T2, and the density of the target material washigher in the second portion T2 than in the third portion T3.

On the other hand, as shown in FIGS. 41B and 43B, when the pulseduration of the pre-pulse laser beam was 10 ns, the droplet diffused ina disc shape or in an annular shape. In this case, the droplet diffusedtoward the direction in which the pre-pulse laser beam travels.

When the pulse duration of the pre-pulse laser beam is in the nanosecondrange, laser ablation from the droplet may occur over a time period inthe nanosecond range. During that time period, heat may be conductedinto the droplet, a part of the droplet may be vaporized, or the dropletmay move due to the reaction of the laser ablation. On the other hand,when the pulse duration of the pre-pulse laser beam is in the picosecondrange, the droplet may be broken up instantaneously before the heat isconducted into the droplet. Such a difference in the diffusion processof the droplet may be a cause for the higher CE with a pre-pulse laserbeam having a small fluence when the pulse duration thereof is in thepicosecond range, compared to the case where the pulse duration thereofis in the nanosecond range (see FIG. 40A).

Further, the particle size of the fine particles of the target materialincluded in the diffused target was smaller when the pulse duration ofthe pre-pulse laser beam was in the picosecond range, compared to thecase where the pulse duration was in the nanosecond range. Accordingly,the diffused target may be turned into plasma more efficiently when thediffused target is irradiated with the main pulse laser beam in a casewhere the pulse duration of the pre-pulse laser beam is in thepicosecond range. This may be a cause for the higher CE when the pulseduration is in the picosecond range, compared to the case where thepulse duration is in the nanosecond range.

15.3.4 Generation Process of Diffused Target

FIGS. 44A through 44C are sectional views schematically illustrating aprocess through which a diffused target is generated when a droplet isirradiated with a pre-pulse laser beam having a pulse duration in thepicosecond range. FIG. 44A shows a presumed state of the target materialafter a time in the picosecond range has passed since the droplet startsto be irradiated with the pre-pulse laser beam having a pulse durationin the picosecond range. FIG. 44B shows a presumed state of the targetmaterial after a time in the nanosecond range has passed since thedroplet starts to be irradiated with the pre-pulse laser beam having apulse duration in the picosecond range. FIG. 44C shows a state of adiffused target after approximately 1 μs has passed since the dropletstarts to be irradiated with the pre-pulse laser beam having a pulseduration in the picosecond range (see FIG. 43A).

As shown in FIG. 44A, when the droplet is irradiated with the pre-pulselaser beam, a part of the energy of the pre-pulse laser beam may beabsorbed into the droplet. As a result, laser ablation, a jet of ions oratoms of the target material, may occur substantially normal to thesurface of the droplet irradiated with the pre-pulse laser beam towardthe outside of the droplet. Then, the reaction of the laser ablation mayact normal onto the surface of the droplet irradiated with the pre-pulselaser beam.

This pre-pulse laser beam may have a fluence equal to or higher than 6.5J/cm², and the irradiation may be completed within the picosecond range.Thus, the energy of the pre-pulse laser beam which the droplet receivesper unit time may be relatively large (see FIG. 40B). Accordingly, alarge amount of laser ablation may occur in a short period of time.Thus, the reaction of the laser ablation may be large, and a shock wavemay occur into the droplet.

The shock wave may travel substantially normal to the surface of thedroplet irradiated with the pre-pulse laser beam, and thus the shockwave may converge at substantially the center of the droplet. Thecurvature of the wavefront of the shock wave may be substantially thesame as that of the surface of the droplet. As the shock wave converges,the energy may be concentrated, and when the concentrated energy exceedsa predetermined level, the droplet may begin to break up.

It is speculated that the break-up of the droplet starts from asubstantially semi-spherical wavefront of the shock wave whose energyhas exceeded the aforementioned predetermined level as the shock waveconverges. This may be a reason why the droplet has diffused in a domeshape in a direction opposite to the direction in which the pre-pulselaser beam has struck the droplet.

When the shock wave converges at the center of the droplet (see FIG.40A), the energy may be at highest concentration, and the remaining partof the droplet may be broken up at once. This may be a reason why thedroplet has diffused in an annular shape in the direction in which thepre-pulse laser beam has struck the droplet, as shown in FIG. 44C.

Although it is speculated that a large amount of laser ablation occursin the state shown in FIG. 44A, the time in which the laser ablationoccurs is short, and the time it takes for the shock wave to reach thecenter of the droplet may also be short. Then, as shown in FIG. 44B, itis speculated that the droplet has already started to break up after atime in the nanosecond range has elapsed. This may be a reason why thecentroid of the diffused target does not differ much from the positionof the center of the droplet prior to being irradiated with thepre-pulse laser beam.

FIGS. 45A through 45C are sectional views schematically illustrating aprocess through which a diffused target is generated when a droplet isirradiated with a pre-pulse laser beam having a pulse duration in thenanosecond range. FIG. 45A shows a presumed state of the target materialafter a time in the picosecond range has passed since the droplet startsto be irradiated with the pre-pulse laser beam having a pulse durationin the nanosecond range. FIG. 45B shows a presumed state of the targetmaterial after a time in the nanosecond range has passed since thedroplet starts to be irradiated with the pre-pulse laser beam having apulse duration in the nanosecond range. FIG. 45C shows a state of adiffused target after a few μs has passed since the droplet starts to beirradiated with the pre-pulse laser beam having a pulse duration in thenanosecond range (see FIG. 43B).

As shown in FIG. 45A, when the droplet is irradiated with the pre-pulselaser beam, a part of the energy of the pre-pulse laser beam may beabsorbed into the droplet. As a result, laser ablation may occursubstantially normal to the surface of the droplet irradiated with thepre-pulse laser beam. Then, the reaction of the laser ablation may actsubstantially normal onto the surface of the droplet irradiated with thepre-pulse laser beam.

This pre-pulse laser beam has a pulse duration in the nanosecond range.This pre-pulse laser beam may have a fluence similar to that of theabove-described pre-pulse laser beam having a pulse duration in thepicosecond range. However, since the droplet is irradiated with thepre-pulse laser beam having a pulse duration in the nanosecond rangeover a time period in the nanosecond range, the energy of the pre-pulselaser beam which the droplet receives per unit time is smaller (see FIG.40B).

A sonic speed V through liquid tin forming the droplet is approximately2500 m/s. When the diameter Dd of the droplet is 21 μm, a time Ts inwhich the sonic wave travels from the surface of the droplet irradiatedwith the pre-pulse laser beam to the center of the droplet may becalculated as follows.

$\begin{matrix}{{Ts} = {\left( {{Dd}/2} \right)/V}} \\{= {\left( {21 \times {10^{- 6}/2}} \right)/2500}} \\{= {4.2\mspace{14mu}{ns}}}\end{matrix}\quad$

In the above-described measurement (see FIGS. 39 through 42), thefluence of the pre-pulse laser beam is not set to be high enough tovaporize the entire droplet as ions or atoms by the laser ablation.Accordingly, when the droplet is irradiated with the pre-pulse laserbeam having a pulse duration of 10 ns, the thickness of the droplet inthe direction in which the pre-pulse laser beam travels may not bereduced more than 21 μm within 10 ns. That is, the speed at which thethickness of the droplet decreases by being pressurized by the reactionof the laser ablation may not exceed the sonic speed in liquid tin.Accordingly, the shock wave may not likely to occur inside the droplet.

The droplet irradiated with such a pre-pulse laser beam having a pulseduration in the nanosecond range may deform into a flat or substantiallydisc shape due to the reaction of the laser ablation acting on thedroplet over a time period in the nanosecond range, as shown in FIG.45B. Then, when the force causing the droplet to deform due to thereaction of the laser ablation overcomes the surface tension, thedroplet may break up. This may be a reason why the droplet has diffusedin a disc shape or in an annular shape as shown in FIG. 45C.

Further, as stated above, the reaction of the laser ablation may act onthe droplet for a time period in the nanosecond range in theabove-described case. Thus, this droplet may be accelerated by thereaction of the laser ablation for an approximately 1000 times longerperiod of time than in a case where the droplet is irradiated with thepre-pulse laser beam having a pulse duration in the picosecond range.This may be a reason why the centroid of the diffused target is shiftedfrom the center of the droplet in the direction in which the pre-pulselaser beam travels, as shown in FIG. 45C.

15.3.5 Range of Pulse Duration

As stated above, when the droplet is irradiated with the pre-pulse laserbeam having a pulse duration in the picosecond range, a shock wave mayoccur inside the droplet and the droplet may break up from the vicinityof the center thereof. On the other hand, when the droplet is irradiatedwith the pre-pulse laser beam having a pulse duration in the nanosecondrange, a shock wave may not occur and the droplet may break up from thesurface thereof.

Based on the above, the conditions for causing a shock wave to occur bythe pre-pulse laser beam and breaking up the droplet may be as follows.Here, the diameter Dd of the droplet may be 10 μm to 40 μm.

When the diameter Dd of the droplet is 40 μm, a time Is required for thesonic wave to reach the center of the droplet from the surface thereofis calculated as follows.

$\begin{matrix}{{Ts} = {\left( {{Dd}/2} \right)/V}} \\{= {\left( {40 \times {10^{- 6}/2}} \right)/2500}} \\{= {8\mspace{14mu}{ns}}}\end{matrix}\quad$

A pulse duration Tp of the pre-pulse laser beam may be sufficientlyshorter than the time Is required for the sonic wave to reach the centerof the droplet from the surface thereof. Irradiating the droplet withthe pre-pulse laser beam having a certain level of fluence within such ashort period of time may cause a shock wave to occur, and the dropletmay break up into fine particles.

A coefficient K will now be defined. The coefficient K may be set todetermine a pulse duration Tp which is sufficiently smaller than thetime Ts required for the sonic wave to reach the center of the dropletfrom the surface thereof. As in Expression (8) below, a value smallerthan a product of the time Ts and the coefficient K may be the pulseduration Tp of the pre-pulse laser beam.Tp<K·Ts  (8)The coefficient K may, for example, be set as K<⅛. In other embodiments,the coefficient K may be set as K≦ 1/16. In yet other embodiments, thecoefficient K may be set as K≦ 1/160.

When the diameter Dd of the droplet is 40 μm, a value for the pulseduration Tp of the pre-pulse laser beam may be induced from Expression(8) above as follows.

When K<⅛, Tp<1 ns

In other embodiments, when K≦ 1/16, Tp≦500 μs

In yet other embodiments, when K≦ 1/160, Tp≦50 μs

15.3.6 Range of Fluence

Referring back to FIG. 40A, when a fluence of the pre-pulse laser beamhaving a pulse duration in the picosecond range is set to be equal to orhigher than 6.5 J/cm², the CE of 3.5% or higher is obtained when thediffused target is irradiated with the main pulse laser beam in theoptimal third delay time. When the fluence is set to be equal to orhigher than 30 J/cm², the CE of 4% or higher is obtained. Further, whenthe fluence is set to be equal to or higher than 45 J/cm², the CE of4.5% or higher is obtained. Accordingly, the fluence of the pre-pulselaser beam having the pulse duration in the picosecond range may be setto be equal to or higher than 6.5 J/cm². In other embodiments, thefluence may be set to 30 J/cm², and in yet other embodiments, thefluence may be set to 45 J/cm².

An energy Ed absorbed by the droplet when the droplet is irradiated withthe pre-pulse laser beam having a pulse duration in the picosecond rangemay be approximated from the following expression.Ed≈F·A·π·(Dd/2)²Here, F is the fluence of the pre-pulse laser beam, and A is anabsorptance of the pre-pulse laser beam by the droplet. When the targetmaterial is liquid tin, and the wavelength of the pre-pulse laser beamis 1.06 μm, A is approximately 16%. Dd is the diameter of the droplet.

Mass m of the droplet may be obtained from the following expression.m=ρ·(4π/3)·(Dd/2)³Here, ρ is the density of the droplet. When the target material isliquid tin, ρ is approximately 6.94 g/cm³.

Then, an energy Edp of the pre-pulse laser beam absorbed by the dropletper unit mass may be obtained from Expression (9) below.

$\begin{matrix}{\begin{matrix}{{Edp} = {{Ed}/m}} \\{\approx {\left( {3/2} \right) \cdot F \cdot {A/\left( {\rho \cdot {Dd}} \right)}}}\end{matrix}\quad} & (9)\end{matrix}$

Accordingly, when the target material is liquid tin and the CE of 3.5%is obtained (i.e., the fluence F of the pre-pulse laser beam is 6.5J/cm²), the energy Edp absorbed by the droplet per unit mass may beobtained from Expression (9) above as follows.

$\begin{matrix}{{Edp} \approx {\left( {3/2} \right) \times 6.5 \times {0.16/\left( {6.94 \times 21 \times 10^{- 4}} \right)}}} \\{\approx {107\mspace{14mu} J\text{/}g}}\end{matrix}\quad$

When the CE of 4% is obtained (i.e., the fluence F of the pre-pulselaser beam is 30 J/cm²), the energy Edp absorbed by the droplet per unitmass may be obtained as follows.

$\begin{matrix}{{Edp} \approx {\left( {3/2} \right) \times 30.0 \times {0.16/\left( {6.94 \times 21 \times 10^{- 4}} \right)}}} \\{\approx {494\mspace{14mu} J\text{/}g}}\end{matrix}$

When the CE of 4.5% is obtained (i.e., the fluence F of the pre-pulselaser beam is 45 J/cm²), the energy Edp absorbed by the droplet per unitmass may be obtained as follows.

$\begin{matrix}{{Edp} \approx {\left( {3/2} \right) \times 45 \times {0.16/\left( {6.94 \times 21 \times 10^{- 4}} \right)}}} \\{\approx {741\mspace{14mu} J\text{/}g}}\end{matrix}$

Further, from Expression (9), the relationship between the fluence F ofthe pre-pulse laser beam and the energy Edp absorbed by the droplet perunit mass may be expressed as follows.F≈(⅔)Edp·ρ·Dd/A

Accordingly, the fluence F of the pre-pulse laser beam to obtain the CEof 3.5% using a given target material may be obtained using theaforementioned Edp as follows.

$\begin{matrix}{F \approx {\left( {2/3} \right){107 \cdot \rho \cdot {{Dd}/A}}}} \\{\approx {71.3\left( {\rho \cdot {{Dd}/A}} \right)}}\end{matrix}\quad$

The fluence F of the pre-pulse laser beam to obtain the CE of 4% using agiven target material may be obtained as follows.

$\begin{matrix}{F \approx {\left( {2/3} \right){494 \cdot \rho \cdot {{Dd}/A}}}} \\{\approx {329\left( {\rho \cdot {{Dd}/A}} \right)}}\end{matrix}\quad$

The fluence F of the pre-pulse laser beam to obtain the CE of 4.5% usinga given target material may be obtained as follows.

$\begin{matrix}{F \approx {\left( {2/3} \right){741 \cdot \rho \cdot {{Dd}/A}}}} \\{\approx {494\left( {\rho \cdot {{Dd}/A}} \right)}}\end{matrix}\quad$

Accordingly, the value of the fluence F of the pre-pulse laser beam maybe equal to or greater than the values obtained as above. Further, thevalue of the fluence F of the pre-pulse laser beam may be equal to orsmaller than the value of the fluence of the main pulse laser beam. Thefluence of the main pulse laser beam may, for example, be 150 J/cm² to300 J/cm².

15.4 Pre-Pulse Laser Apparatus

15.4.1 General Configuration

A mode-locked laser device may be used to generate a pre-pulse laserbeam having a short pulse duration. The mode-locked laser device mayoscillate at a plurality of longitudinal modes with fixed phases amongone another. When the plurality of longitudinal modes interferes withone another, a pulse of a laser beam having a short pulse duration maybe outputted. However, a timing at which a given pulse of the pulselaser beam is outputted from the mode-locked laser device may depend ona timing at which a preceding pulse is outputted and a repetition ratein accordance with a resonator length of the mode-locked laser device.Accordingly, it may not be easy to control the mode-locked laser devicesuch that each pulse is outputted at a desired timing. Thus, in order tocontrol the timing at which a droplet supplied into the chamber 1 isirradiated with a given pulse of a pre-pulse laser beam, a pre-pulselaser apparatus may be configured as follows.

FIG. 46 schematically illustrates an exemplary configuration of apre-pulse laser apparatus shown in FIG. 38. The pre-pulse laserapparatus 300 may include a clock generator 301, a mode-locked laserdevice 302, a resonator length adjusting driver 303, a pulse laser beamdetector 304, a regenerative amplifier 305, an excitation power supply306, and a controller 310.

The clock generator 301 may, for example, output a clock signal at arepetition rate of 100 MHz. The mode-locked laser device 302 may outputa pulse laser beam at a repetition rate of approximately 100 MHz, forexample. The mode-locked laser device 302 may include a resonator, whichwill be described later, and the resonator length thereof may beadjusted through the resonator length adjusting driver 303.

A beam splitter 307 may be provided in a beam path of the pulse laserbeam from the mode-locked laser device 302. The pulse laser beam may besplit by the beam splitter 307, and the pulse laser beam detector 304may be provided in a beam path of a part of the pulse laser beam splitby the beam splitter 307. The pulse laser beam detector 304 may beconfigured to detect the pulse laser beam and output a detection signal.

The regenerative amplifier 305 may be provided in a beam path of theother part of the pulse laser beam split by the beam splitter 307. Thedetails of the regenerative amplifier 305 will be given later.

The controller 310 may include a phase adjuster 311 and an AND circuit312. The phase adjuster 311 may carry out a feedback-control on theresonator length adjusting driver 303 based on the clock signal from theclock generator 301 and the detection signal from the pulse laser beamdetector 304.

Further, the controller 310 may control the regenerative amplifier 305based on the clock signal from the clock generator 301 and theaforementioned first timing signal from the delay circuit 153 describedwith reference to FIG. 38. More specifically, the AND circuit 312 may beconfigured to generate an AND signal of the clock signal and the firsttiming signal, and control a Pockels cell inside the regenerativeamplifier 305 through the AND signal of the clock signal.

15.4.2 Mode-Locked Laser Device

FIG. 47 schematically illustrates an exemplary configuration of amode-locked laser device shown in FIG. 46. The mode-locked laser device302 may include a resonator formed by a flat mirror 320 and a saturableabsorber mirror 321, and a laser crystal 322, a concave mirror 323, aflat mirror 324, an output coupler mirror 325, and a concave mirror 326are provided in this order from the side of the flat mirror 320 in abeam path in the resonator. The beam path in the resonator may besubstantially parallel to the paper plane. The mode-locked laser device302 may further include an excitation light source 327 configured tointroduce excitation light E1 to the laser crystal 322 from the outsideof the resonator. The excitation light source 327 may include a laserdiode to generate the excitation light E1.

The flat mirror 320 may be configured to transmit the excitation lightE1 from the excitation light source 327 with high transmittance andreflect light from the laser crystal 322 with high reflectance. Thelaser crystal 322 may be a laser medium that undergoes stimulatedemission with the excitation light E1. The laser crystal 322 may, forexample, be a neodymium-doped yttrium orthovanadate (Nd:YVO₄) crystal.Light emitted from the laser crystal 322 may include a plurality oflongitudinal modes. The laser crystal 322 may be arranged so that alaser beam is incident on the laser crystal 322 at a Brewster's angle.

The concave mirror 323, the flat mirror 324, and the concave mirror 326may reflect the light emitted from the laser crystal 322 with highreflectance. The output coupler mirror 325 may be configured to transmita part of the laser beam amplified in the laser crystal 322 to theoutside of the resonator and reflect the remaining part of the laserbeam back into the resonator to be further amplified in the lasercrystal 322. First and second laser beams that travel in differentdirections may be outputted through the output coupler mirror 325 to theoutside of the resonator. The first laser beam is a part of the laserbeam reflected by the flat mirror 324 and transmitted through the outputcoupler mirror 325, and the second laser beam is a part of the laserbeam reflected by the concave mirror 326 and transmitted through theoutput coupler mirror 325. The aforementioned beam splitter 307 may beprovided in a beam path of the first laser beam, and a beam dump (notshown) may be provided in a beam path of the second laser beam.

The saturable absorber mirror 321 may be formed such that a reflectivelayer is laminated on a mirror substrate and a saturable absorber layeris laminated on the reflective layer. In the saturable absorber mirror321, the saturable absorber layer may absorb an incident ray while theintensity thereof is equal to or lower than a predetermined thresholdvalue. When the intensity of the incident ray exceeds the predeterminedthreshold value, the saturable absorber layer may transmit the incidentray and the reflective layer may reflect the incident ray. With thisconfiguration, only high-intensity pulses of the laser beam may bereflected by the saturable absorber mirror 321. The high-intensitypulses may be generated when the plurality of longitudinal modes is inphase with one another.

In this way, the mode-locked pulses of the laser beam may travel backand forth in the resonator and be amplified. The amplified pulses may beoutputted through the output coupler mirror 325 as a pulse laser beam.The repetition rate of this pulse laser beam may correspond to aninverse of a time it takes for a pulse to make a round trip in theresonator. For example, when the resonator length L is 1.5 m, the speedof light in vacuum c is 3×10⁸ m/s, a refractive index in the beam path,which is obtained by dividing the speed of light in vacuum by the speedof light in a material in the beam path, is 1, a repetition rate f maybe 100 MHz as obtained from the following expression.

$\begin{matrix}{f = {c/\left( {2L} \right)}} \\{= {\left( {3 \times 10^{8}} \right)/\left( {2 \times 1.5} \right)}} \\{= {100\mspace{14mu}{MHz}}}\end{matrix}\quad$Since the laser crystal 322 is arranged at a Brewster's angle to thelaser beam, the pulse laser beam outputted from the mode-locked laserbeam 302 may be a linearly polarized laser beam whose polarizationdirection is parallel to the paper plane.

The saturable absorber mirror 321 may be held by a mirror holder, andthis mirror holder may be movable by a linear stage 328 in the directionin which the laser beam is incident on the saturable absorber mirror321. The linear stage 328 may be driven through the aforementionedresonator length adjusting driver 303. As the saturable absorber mirror321 is moved in the direction in which the laser beam is incident on thesaturable absorber mirror 321, the resonator length may be adjusted toadjust the repetition rate of the pulse laser beam.

As mentioned above, the phase adjuster 311 may be configured to controlthe resonator length adjusting driver 303 based on the clock signal fromthe clock generator 301 and the detection signal from the pulse laserbeam detector 304. More specifically, the phase adjuster 311 may detecta phase difference between the clock signal and the detection signal,and control the resonator length adjusting driver 303 so that the clocksignal and the detection signal are in synchronization with a certainphase difference, a fourth delay time. The fourth delay time will bedescribed later with reference to FIGS. 50A and 50B.

15.4.3 Regenerative Amplifier

FIG. 48 schematically illustrates an exemplary configuration of theregenerative amplifier shown in FIG. 46. The regenerative amplifier 305may include a resonator formed by a flat mirror 334 and a concave mirror335, and a laser crystal 336, a concave mirror 337, a flat mirror 338, apolarization beam splitter 339, a Pockels cell 340, and a quarter-waveplate 341 may be provided in this order from the side of the flat mirror334 in a beam path in the resonator. The resonator length of theresonator in the regenerative amplifier 305 may be shorter than that ofthe resonator in the mode-locked laser device 302. Further, theregenerative amplifier 305 may include an excitation light source 342configured to introduce excitation light E2 to the laser crystal 336from the outside of the resonator. An electric power may be supplied tothe excitation light source 342 from the excitation power supply 306.The excitation light source 342 may include a laser diode to generatethe excitation light E2. Further, the regenerative amplifier 305 mayinclude a polarization beam splitter 330, a Faraday optical isolator331, and flat mirrors 332 and 333. The Faraday optical isolator 331 mayinclude a Faraday rotator (not shown) and a quarter-wave plate (notshown).

The flat mirror 334 may be configured to transmit the excitation lightE2 from the excitation light source 342 with high transmittance andreflect light emitted from the laser crystal 336 with high reflectance.The laser crystal 336 may be a laser medium excited by the excitationlight E2, and may, for example, be a neodymium-doped yttrium aluminumgarnet (Nd:YAG) crystal. Further, the laser crystal 336 may be arrangedso that a laser beam is incident on the laser crystal 336 at aBrewster's angle. When a seed beam outputted from the mode-locked laserdevice 302 is incident on the laser crystal 336 excited by theexcitation light E2, the seed beam may be amplified through stimulatedemission.

15.4.3.1 When Voltage is not Applied to Pockels Cell

The beam splitter 330 may be provided in a beam path of a pulse laserbeam B1 from the mode-locked laser device 302. The polarization beamsplitter 330 may, for example, be arranged such that light receivingsurfaces thereof are perpendicular to the paper plane. The polarizationbeam splitter 330 may be configured to transmit a polarization componentparallel to the paper plane with high transmittance and reflect theother polarization component perpendicular to the paper plane with highreflectance.

The Faraday optical isolator 331 may be provided in a beam path of apulse laser beam B2 transmitted through the polarization beam splitter330. The Faraday optical isolator 331 may shift a phase differencebetween the two polarization components of the incident pulse laser beamB2 by 180 degrees and output as a pulse laser beam B3. That is, theFaraday optical isolator 331 may rotate the polarization direction ofthe incident linearly polarized laser beam B2 by 90 degrees. Further,the Faraday optical isolator 331 may transmit a pulse laser beam B28,which will be described later, toward the polarization beam splitter 330without rotating the polarization direction thereof.

The flat mirror 322 may be provided in a beam path of the pulse laserbeam B3 transmitted through the Faraday optical isolator 331. The flatmirror 332 may reflect the pulse laser beam B3 with high reflectance.The flat mirror 333 may reflect a pulse laser beam B4 reflected by theflat mirror 332 with high reflectance.

The polarization beam splitter 339 in the resonator may be provided in abeam path of a pulse laser beam B5 reflected by the flat mirror 333. Thepolarization beam splitter 339 may be provided such that the lightreceiving surfaces thereof are perpendicular to the paper plane, and thepulse laser beam B5 may be incident on a first surface of thepolarization beam splitter 339. The polarization beam splitter 339 mayreflect the linearly polarized pulse laser beam B5 polarized in adirection perpendicular to the paper plane with high reflectance tothereby guide into the resonator as a pulse laser beam B6.

A voltage may be applied to the Pockels cell 340 by a high-voltage powersupply 343. However, when the voltage is not applied to the Pockels cell340, the Pockels cell 340 may transmit the entering pulse laser beam B6without shifting the phase difference between the two polarizationcomponents thereof.

The quarter-wave plate 341 may shift a phase difference between the twopolarization components of a pulse laser beam B7 by 90 degrees. Theconcave mirror 335 may reflect a pulse laser beam B8 from thequarter-wave plate 341 with high reflectance. A pulse laser beam B9reflected by the concave mirror 335 may be transmitted through thequarter-wave plate 341, and the phase difference between the twopolarization components thereof may be shifted by 90 degrees. In thisway, the pulse laser beam B9 may be transformed into a linearlypolarized pulse laser beam B10 polarized in a direction parallel to thepaper plane.

As stated above, when the voltage is not applied to the Pockels cell340, the Pockels cell 340 may transmit the incident pulse laser beamwithout shifting the phase difference between the two polarizationcomponents. Accordingly, a pulse laser beam B11 transmitted through thePockels cell 340 may be incident on the first surface of thepolarization beam splitter 339 as a linearly polarized pulse laser beampolarized in a direction parallel to the paper plane and be transmittedthrough the polarization beam splitter 339 with high transmittance.

The flat mirror 338 may reflect a pulse laser beam B12 from thepolarization beam splitter 339 with high reflectance. The concave mirror337 may reflect a pulse laser beam B13 from the flat mirror 338 withhigh reflectance. A pulse laser beam B14 from the concave mirror 337 maythen be incident on the laser crystal 336, and be amplified in the lasercrystal 336.

The flat mirror 334 may reflect a pulse laser beam B15 from the lasercrystal 336 with high reflectance back to the laser crystal 336 as apulse laser beam B16. A pulse laser beam B17 amplified by the lasercrystal 336 may be reflected by the concave mirror 337 as a pulse laserbeam B18. The pulse laser beam B18 may then be reflected the flat mirror338, and, as a pulse laser beam B19, transmitted through thepolarization beam splitter 339. A pulse laser beam B20 from the beamsplitter 339 may enter the Pockels cell 340, and be incident on thequarter-wave plate 341 as a pulse laser beam B21. The pulse laser beamB21 may be transmitted through the quarter-wave plate 341, and, as apulse laser beam B22, reflected by the concave mirror 335. A pulse laserbeam B23 may then be transmitted again through the quarter-wave plate341, to thereby be converted into a linearly polarized pulse laser beamB24 polarized in a direction perpendicular to the paper plane. The pulselaser beam B24 may be transmitted through the Pockels cell 340,reflected, as a pulse laser beam B25, by the polarization beam splitter339, and outputted as a pulse laser beam B26 to the outside of theresonator.

The pulse laser beam B26 may be reflected by the high-reflection mirror333, and, as a pulse laser beam B27, reflected by the high-reflectionmirror 332. Then, a pulse laser beam 28 from the high-reflection mirror332 may enter the Faraday optical isolator 331. As stated above, theFaraday optical isolator 331 may transmit the linearly polarized pulselaser beam B28 as a linearly polarized pulse laser beam B29 withoutrotating the polarization direction thereof. The polarization beamsplitter 330 may reflect the linearly polarized pulse laser beam B29polarized in a direction perpendicular to the paper plane with highreflectance.

A pulse laser beam B30 reflected by the polarization beam splitter 330may be guided to the plasma generation region PS through the laser beamfocusing optical system 122 (see FIG. 38). Here, even when a droplet isirradiated with the pulse laser beam B30 outputted after making only oneround trip in the resonator in the regenerative amplifier 305, thedroplet may not be diffused. This pulse laser beam B30 may not have abeam intensity sufficient to turn the droplet into plasma.

15.4.3.2 When Voltage is Applied to Pockels Cell

The high-voltage power supply 343 may apply a voltage to Pockels cell340 at a given timing prior to the pulse laser beam B20 entering thePockels cell 340. When the voltage is applied to the Pockels cell 340,the Pockels cell 340 may shift the phase difference between the twopolarization components of the entering pulse laser beam by 90 degrees.

FIG. 49 schematically illustrates a beam path in the regenerativeamplifier shown in FIG. 48 when a voltage is applied to the Pockelscell. Here, the pulse laser beam B20 may be transmitted through thePockels cell 340 twice and the quarter-wave plate 341 twice, asindicated by pulse laser beams Ba1, Ba2, Ba3, and Ba4, and return as thepulse laser beam B11. The pulse laser beam B11 that has been transmittedthrough the quarter-wave plate 341 twice and transmitted through thePockels cell 340 twice to which the voltage is applied may have itspolarization direction oriented toward the same direction as that of thepulse laser beam B20. Accordingly, the pulse laser beam B11 may betransmitted through the polarization beam splitter 339 and be amplifiedby the laser crystal 336. While the voltage is applied to the Pockelscell 340, this amplification operation may be repeated.

After the amplification operation is repeated, the high-voltage powersupply 343 may set the voltage applied to the Pockels cell 340 to OFF ata given timing prior to the pulse laser beam B20 entering the Pockelscell 340. As stated above, when the voltage is not applied to thePockels cell 340 from the high-voltage power supply 343, the Pockelscell 340 may not shift the phase difference between the two polarizationcomponents of the entering pulse laser beam. Accordingly, the pulselaser beam B20 entering the Pockels cell 340 when the voltage is notapplied thereto may have its polarization direction rotated only by 90degrees as it is transmitted through the quarter-wave plate 341 twice(see the pulse laser beams B21, B22, B23, and B24 shown in FIG. 48).Thus, the pulse laser beam after the amplification operation is repeatedmay be incident on the first surface of the polarization beam splitter339 as the linearly polarized pulse laser beam B25 polarized in adirection perpendicular to the paper plane and be outputted to theoutside of the resonator.

While the voltage is applied to the Pockels cell 340 and theamplification operation is repeated (see FIG. 49), the pulse laser beamB1 newly outputted from the mode-locked laser device 302 may enter thePockels cell 340 as the linearly polarized pulse laser beam B6 polarizedin a direction perpendicular to the paper plane. While the voltage isapplied to the Pockels cell 340, the pulse laser beam B6 may betransmitted through the quarter-wave plate 341 twice and the Pockelscell 340 twice (see pulse laser beams Ba5, Ba6, Ba1, and Ba8) and returnas the pulse laser beam B25. Here, the pulse laser beam B25 may have itspolarization direction oriented to the same direction as that of thepulse laser beam B6. Accordingly, the pulse laser beam B25 may bereflected by the first surface of the polarization beam splitter 339,and as a pulse laser beam B26, outputted to the outside of the resonatorwithout being amplified even once.

A timing at which the high-voltage power supply 343 sets the voltageapplied to the Pockels cell 340 to ON/OFF may be determined by the ANDsignal of the clock signal and the first timing signal described above.The AND signal may be supplied to the voltage waveform generationcircuit 344 in the regenerative amplifier 305 from the AND circuit 312.The voltage waveform generation circuit 344 may generate a voltagewaveform with the AND signal as a trigger, and supply this voltagewaveform to the high-voltage power supply 343. The high-voltage powersupply 343 may generate a pulse voltage in accordance with the voltagewaveform and apply this pulse voltage to the Pockels cell 340. The firsttiming signal, the AND signal, and the voltage waveform by the voltagewaveform generation circuit 344 will be described later with referenceto FIGS. 50C through 50E.

15.4.4 Timing Control

FIGS. 50A through 50E show timing charts of various signals in thepre-pulse laser apparatus shown in FIG. 46. FIG. 50A is a timing chartof the clock signal outputted from the clock generator. The clockgenerator 301 may, for example, be configured to output the clock signalat a repetition rate of 100 MHz. In this case, the interval of thepulses may be 10 ns.

FIG. 50B is a timing chart of a detection signal outputted from thepulse laser beam detector. A repetition rate of the detection signal maydepend on the repetition rate of the pulse laser beam outputted from themode-locked laser device 302. The repetition rate of the pulse laserbeam from the mode-locked laser device 302 may be adjusted by adjustingthe resonator length of the mode-locked laser device 302. In thisexample, the repetition rate of the pulse laser beam may beapproximately 100 MHz. By fine-tuning the repetition rate of the pulselaser beam, the phase difference from the clock signal shown in FIG. 50Amay be adjusted. Thus, a feedback-control may be carried out on themode-locked laser device 302 so that the detection signal of the pulselaser beam is in synchronization with the clock signal shown in FIG. 50Awith the fourth delay time of, for example, 5 ns.

FIG. 50C is a timing chart of the first timing signal outputted from thedelay circuit. As stated above, the first timing signal from the delaycircuit 153 may be a signal in which the first delay time is given tothe target detection signal by the target sensor 104. A repetition rateof the first timing signal may depend on the repetition rate of thedroplets outputted from the target supply unit 2. The droplets may, forexample, be outputted from the target supply unit 2 at a repetition rateof approximately 100 kHz. The pulse duration of the first timing signalmay be 10 ns.

FIG. 50D is a timing chart of the AND signal outputted from the ANDcircuit. The AND signal from the AND circuit 312 may be a signal of alogical product of the clock signal and the first timing signal. Whenthe pulse duration of the first timing signal is substantially the sameas the interval of the clock signal, such as 10 ns, a single pulse ofthe AND signal may be generated for a single pulse of the first timingsignal. The AND signal may be generated to be substantially insynchronization with a part of multiple pulses of the clock signal.

FIG. 50E is a timing chart of the voltage waveform outputted from thevoltage waveform generation circuit. The voltage waveform from thevoltage waveform generation circuit 344 may be generated atsubstantially the same time as the AND signal from the AND circuit 312.The voltage waveform may, for example, have a pulse duration of 300 ns.For example, when the resonator length of the regenerative amplifier 305is 1 m, the pulse laser beam makes 50 round trips in the resonator in300 ns at the speed of light of 3×10⁸ m/s. By setting a pulse durationof the voltage waveform, the number of round trips the pulse laser beammakes in the resonator in the regenerative amplifier 305 may be set.

With the above timing control, the clock signal and the pulse laser beamfrom the mode-locked laser device 302 may be in synchronization witheach other with the fourth delay time, and the AND signal may be insynchronization with a part of the pulses of the clock signal. Thus,while the pulse laser beam travels in a specific section of theresonator in the regenerative amplifier 305, the voltage applied to thePockels cell 340 from the high-voltage power supply 343 may be set toON/OFF. Accordingly, only a desired pulse in the pulse laser beam fromthe mode-locked laser device 302 may be amplified to a desired beamintensity, and outputted to strike a droplet.

Further, with the above-described timing control, the timing of a pulsefrom the regenerative amplifier 305 may be controlled with a resolvingpower in accordance with the interval of the pulses from the mode-lockedlaser device 302. For example, a droplet outputted from the targetsupply unit 2 and traveling inside the chamber 1 at a speed of 30 m/s to60 m/s may move 0.3 μm to 0.6 μm in 10 ns, which is the interval of thepulses from the mode-locked laser device 302. When the diameter of thedroplet is 20 μm, the resolving power of 10 ns is sufficient toirradiate the droplet with the pulse laser beam.

15.4.5 Examples of Laser Medium

In the above-described example, an Nd:YVO₄ crystal is used as the lasercrystal 322 in the mode-locked laser device 302, and an Nd:YAG crystalis used as the laser crystal 336 in the regenerative amplifier 305.However, this disclosure is not limited to these crystals.

As one example, an Nd:YAG crystal may be used as a laser crystal in eachof the mode-locked laser device 302 and the regenerative amplifier 305.

As another example, a Titanium-doped Sapphire (Ti:Sapphire) crystal maybe used as a laser crystal in each of the mode-locked laser device 302and the regenerative amplifier 305.

As yet another example, a ruby crystal may be used as a laser crystal ineach of the mode-locked laser device 302 and the regenerative amplifier305.

As yet another example, a dye cell may be used as a laser medium in eachof the mode-locked laser device 302 and the regenerative amplifier 305.

As still another example, a triply ionized neodymium-doped glass(Nd³⁺:glass) may be used as a laser medium in each of the mode-lockedlaser device 302 and the regenerative amplifier 305.

15.5 Main Pulse Laser Apparatus

FIG. 51 schematically illustrates an exemplary configuration of a mainpulse laser apparatus shown in FIG. 38. The main pulse laser apparatus390 may include a master oscillator MO, amplifiers PA1, PA2, and PA3,and a controller 391.

The master oscillator MO may be a CO₂ laser apparatus in which a CO₂ gasis used as a laser medium, or may be a quantum cascade laser apparatusconfigured to oscillate in a bandwidth of the CO₂ laser apparatus. Theamplifiers PA1, PA2, and PA3 may be provided in series in a beam path ofa pulse laser beam outputted from the master oscillator MO. Each of theamplifiers PA1, PA2, and PA3 may include a laser chamber (not shown)filled with a CO₂ gas serving as a laser medium, a pair of electrodes(not shown) provided inside the laser chamber, and a power supply (notshown) configured to apply a voltage between the pair of electrodes.

The controller 391 may be configured to control the master oscillator MOand the amplifiers PA1, PA2, and PA3 based on a control signal from theEUV light generation controller 151. The controller 391 may output theaforementioned second timing signal from the delay circuit 153 to themaster oscillator MO. The master oscillator MO may output each pulse ofthe pulse laser beam in accordance with the second timing signal servingas triggers. The pulse laser beam may be amplified in the amplifiersPA1, PA2, and PA3. Thus, the main pulse laser apparatus 390 may outputthe main pulse laser beam in synchronization with the second timingsignal from the delay circuit 153.

16. Eleventh Embodiment

FIG. 52 is a partial sectional view schematically illustrating anexemplary configuration of an EUV light generation system according toan eleventh embodiment of this disclosure. The EUV light generationsystem according to the eleventh embodiment may include beam splitters161 and 162, optical sensors 163 and 164, a delay time calculation unit165, and a delay time control device 150. Other points may be similar tothose of the tenth embodiment.

The beam splitter 161 may be provided in a beam path of the pre-pulselaser beam and the main pulse laser beam between the dichroic mirror 354and the laser beam focusing optical system 122. The beam splitter 161may be coated with a film configured to transmit the pre-pulse laserbeam and the main pulse laser beam with high transmittance and reflect apart of the pre-pulse laser beam and the main pulse laser beam.

The beam splitter 162 may be provided in a beam path of the pre-pulselaser beam and the main pulse laser beam reflected by the beam splitter161. The beam splitter 162 may be coated with a film configured toreflect the pre-pulse laser beam with high reflectance and transmit themain pulse laser beam with high transmittance.

The optical sensor 163 may be provided in a beam path of the pre-pulselaser beam reflected by the beam splitter 162. The optical sensor 164may be provided in a beam path of the main pulse laser beam transmittedthrough the beam splitter 162. The optical sensors 163 and 164 may beprovided such that the respective optical lengths from the beam splitter162 are equal to each other. The optical sensor 163 may detect thepre-pulse laser beam and output a detection signal. The optical sensor163 may include a fast-response photodiode configured to detect thepre-pulse laser beam at a wavelength of 1.06 μm. The optical sensor 164may detect the main pulse laser beam and output a detection signal. Theoptical sensor 164 may include a fast-response thermoelectric elementconfigured to detect the main pulse laser beam at a wavelength of 10.6μm.

The delay time calculation unit 165 may be connected to the opticalsensors 163 and 164 through respective signal lines. The delay timecalculation unit 165 may receive detection signals from the respectiveoptical sensors 163 and 164, and calculate a delay time δT from thedetection of the pre-pulse laser beam to the detection of the main pulselaser beam based on the received detection signals. Here, the calculateddelay time δT may be equivalent to the aforementioned third delay time,and thus this delay time δT will serve as the third delay timehereinafter. The delay time calculation unit 165 may output thecalculated third delay time δT to the delay time control device 150.

FIG. 53 schematically illustrates an exemplary configuration of a delaytime control device shown in FIG. 52. The delay time control device 150may include the delay circuit 153 and a controller 154. The delaycircuit 153 may output to the pre-pulse laser apparatus 300 the firsttiming signal in which the first delay time is given to the targetdetection signal outputted from the droplet controller 152. Further, thedelay circuit 153 may output to the main pulse laser apparatus 390 thesecond timing signal having the second delay time δTo from the firsttiming signal. The second delay time δTo may vary.

The controller 154 may receive a target value δTt of the third delaytime from the EUV light generation controller 151. Further, thecontroller 154 may receive the calculated third delay time δT from thedelay time calculation unit 165. The controller 154 may be configured tocontrol the delay circuit 153 to modify the second delay time δTo basedon a difference between the third delay time δT and the target valueδTt.

FIG. 54 is a flowchart showing an exemplary operation of the controllershown in FIG. 53. The controller 154 may carry out a feedback-control onthe delay circuit 153 based on the difference between the third delaytime δT and the target value δTt.

The controller 154 may first receive an initial value of a delayparameter α from the EUV light generation controller 151 (Step S1). Theinitial value of the delay parameter α may be calculated from thefollowing expression.α=(Lm−Lp)/cHere, Lm may be a beam path length of the main pulse laser beam from themaster oscillator MO (see FIG. 51) of the main pulse laser apparatus 390to the plasma generation region PS, Lp may be a beam path length of thepre-pulse laser beam from the regenerative amplifier 305 (see FIG. 46)of the pre-pulse laser apparatus 300 to the plasma generation region PS,and c may be the speed of light (3×10⁸ m/s).

The main pulse laser apparatus 390 may include a larger number ofamplifiers than the pre-pulse laser apparatus 300 in order to output themain pulse laser beam having a higher beam intensity than the pre-pulselaser beam. Accordingly, the beam path length Lm of the main pulse laserbeam may be longer than the beam path length Lp of the pre-pulse laserbeam, and thus the delay parameter α may be greater than 0.

Then, the controller 154 may receive a target value δTt of the thirddelay time from the EUV light generation controller 151 (Step S2). Thecontroller 154 may then calculate the second delay time δTo bysubtracting the delay parameter α from the target value δTt (Step S3).Subsequently, the controller 154 may send the calculated second delaytime δTo to the delay circuit 153 (Step S4).

Thereafter, the controller 154 may determine whether or not thepre-pulse laser apparatus 300 and the main pulse laser apparatus 390have oscillated (Step S5). When either of these laser apparatuses hasnot oscillated (Step S5; NO (N)), the controller 154 may stand by untilthese laser apparatuses oscillate. When both laser apparatuses haveoscillated (Step S5; YES (Y)), the processing may proceed to Step S6.

Then, the controller 154 may receive the calculated third delay time δTfrom the delay time calculation unit 165 (Step S6). The controller 154may then calculate a difference ΔT between the third delay time δT andthe target value δTt through the following expression (Step S7).ΔT=δT−δTt

Subsequently, the controller 154 may update the delay parameter α byadding the difference ΔT between the third delay time δT and the targetvalue δTt to the delay parameter α (Step S8). That is, when the thirddelay time δT is greater than the target value δTt (ΔT>0), the delayparameter α may be increased by AT so that the second delay time ΔTobecomes smaller.

Thereafter, the controller 154 may determine whether or not thefeedback-control on the delay circuit 153 is to be stopped (Step S9).For example, when the output of the pulse laser beam is to be stoppedbased on a control signal from the EUV light generation controller 151,the feedback-control on the delay circuit 153 may be stopped.Alternatively, when the output energy of the EUV light reaches orexceeds a predetermined value as a result of repeating Steps S2 throughS8 multiple times, the feedback-control on the delay circuit 153 may bestopped and the second delay time δTo may be fixed to generate the EUVlight. When the feedback-control on the delay circuit 153 is not to bestopped (Step S9; NO), the processing may return to Step S2, and thecontroller 154 may receive the target value δTt of the third delay timeand carry out the feedback-control on the delay circuit 153. When thefeedback-control on the delay circuit 153 is to be stopped (Step S9;YES), the processing in this example may be terminated.

As described above, by carrying out the feedback-control on the delaycircuit 153 based on the calculated third delay time δT, the third delaytime δT may be stabilized with high precision. As a result, the diffusedtarget may be irradiated with the main pulse laser beam at an optimalthird delay time, and a CE may be improved. Further, even in a casewhere the third delay time δT varies for some reason although the seconddelay time δTo is fixed, the feedback-control may allow the third delaytime δT to be stabilized.

In the eleventh embodiment, the feedback-control may be carried out onthe delay circuit based on the calculated third delay time. However,this disclosure is not limited thereto, and the third delay time may notbe calculated. For example, the second delay time δTo may be calculatedfrom the initial value of the aforementioned delay parameter α and theaforementioned target value δTt, and the delay circuit 153 may becontrolled based on this second delay time δTo.

The above-described embodiments and the modifications thereof are merelyexamples for implementing this disclosure, and this disclosure is notlimited thereto. Making various modifications according to thespecifications or the like is within the scope of this disclosure, andother various embodiments are possible within the scope of thisdisclosure. For example, the modifications illustrated for particularones of the embodiments can be applied to other embodiments as well(including the other embodiments described herein).

The terms used in this specification and the appended claims should beinterpreted as “non-limiting.” For example, the terms “include” and “beincluded” should be interpreted as “including the stated elements butnot limited to the stated elements.” The term “have” should beinterpreted as “having the stated elements but not limited to the statedelements.” Further, the modifier “one (a/an)” should be interpreted as“at least one” or “one or more.”

What is claimed is:
 1. A system comprising: a laser system configured togenerate a pre-pulse laser beam with a pulse duration of less than 1 ns,and a main pulse laser beam; a target supply unit configured to supply atarget material to be irradiated with the pre-pulse laser beam and themain pulse laser beam for generating extreme ultraviolet light; and alaser controller configured to control the laser system such that afluence of the pre-pulse laser beam is lower than a fluence of the mainpulse laser beam and a beam intensity of the pre-pulse laser beam ishigher than a beam intensity of the main pulse laser beam.
 2. The systemaccording to claim 1, wherein the controller is configured to controlthe laser system such that a pulse energy of the pre-pulse laser beam is0.25 mJ or higher and 2 mJ or lower.
 3. The system according to claim 1,wherein the controller is configured to control the laser system suchthat a delay time from irradiation of the target material with thepre-pulse laser beam to irradiation of the target material with the mainpulse laser beam is 0.5 μs or longer and 1.8 μs or shorter.
 4. Thesystem according to claim 1, wherein the controller is configured tocontrol the laser system such that a delay time from irradiation of thetarget material with the pre-pulse laser beam to irradiation of thetarget material with the main pulse laser beam is 0.7 μs or longer and1.6 μs or shorter.
 5. The system according to claim 1, wherein thecontroller is configured to control the laser system such that a delaytime from irradiation of the target material with the pre-pulse laserbeam to irradiation of the target material with the main pulse laserbeam is 1.0 μs or longer and 1.4 μs or shorter.
 6. The system accordingto claim 1, wherein the controller is configured to control the lasersystem such that a fluence of the pre-pulse laser beam is 2.6 J/cm² orhigher and 6.5 J/cm² or lower.
 7. The system according to claim 1,wherein the controller is configured to control the laser system suchthat a beam intensity of the pre-pulse laser beam is 2.6×10¹¹ W/cm² orhigher and 5.6×10¹¹ W/cm² or lower.
 8. The system according to claim 1,wherein the controller is configured to control the laser system suchthat a pulse energy of the main pulse laser beam is 135 mJ or higher and170 mJ or lower.
 9. The system according to claim 1, wherein thecontroller is configured to control the laser system such that a fluenceof the main pulse laser beam is 150 J/cm² or higher and 300 J/cm² orlower.
 10. The system according to claim 1, wherein the controller isconfigured to control the laser system such that a fluence of the mainpulse laser beam is 191 J/cm² or higher and 241 J/cm² or lower.
 11. Thesystem according to claim 1, wherein the controller is configured tocontrol the laser system such that a beam intensity of the main pulselaser beam is 1.00×10¹⁰ W/cm² or higher and 2.00×10¹⁰ W/cm² or lower.12. The system according to claim 1, wherein the controller isconfigured to control the laser system such that a beam intensity of themain pulse laser beam is 1.27×10¹⁰ W/cm² or higher and 1.60×10¹⁰ W/cm²or lower.
 13. The system according to claim 1, wherein the controller isconfigured to control the laser system such that a pulse energy of thepre-pulse laser beam is lower than a pulse energy of the main pulselaser beam.
 14. The system according to claim 1, wherein a spot size ofthe main pulse laser beam is larger than a spot size of the pre-pulselaser beam, and the spot size of the pre-pulse laser beam is larger thana diameter of a target including the target material.
 15. The systemaccording to claim 1, wherein the laser system includes a plurality oflaser apparatuses.